This patch introduces mem_cast_explicit(). This function is used to
convert a MEM to a given field type according to explicit cast rules.

Part of #5818

This patch introduces mem_to_str() and mem_to_str0() functions. These
functions are used to convert a MEM to a MEM that contains string value.
These functions defines the rules that are used during convertion from
values of all other types to STRING.

Part of #5818

This patch introduces mem_to_number(). This function is used to convert
MEM to MEM contains number.

Part of #5818

This patch intruduces mem_to_double(). This function is used to convert
a MEM to a MEM that contains double value. This function defines the
rules that are used during conversion from values of all other types to
double.

Part of #5818

This patch introduces mem_to_int() and mem_to_int_precise() functions.
These functions are used to convert a MEM to a MEM that contains
integer value. These functions defines the rules that are used during
convertion from values of all other types to INTEGER.

Part of #5818

This patch introduces mem_set_null_clear() function. This function sets
"cleared" NULL to MEM. This NULL is used only in internal VDBE
operations.

Part of #5818

This patch introduces mem_set_agg() function. This function stores
aggregation function to MEM and allocates enough memory to hold
accumulation structure for aggregate function.

Part of #5818

This patch introduces mem_set_frame() function. This function clears the
MEM and sets a frame to MEM. Frames used for internal VDBE operations.

Part of #5818

This patch refactors mem_set_ptr() function. Also, it moves the function
to all others mem_set_*() functions.

Part of #5818

This patch introduces mem_set_invalid() function. This function clears
MEM and invalidates it.

Part of #5818

This patch introduces set of mem_set_*() functions for MAP and ARRAY.
These functions clears MEM and sets it to given binary value. Binary
value must be MAP or ARRAY. Degree of clearing and type of allocation of
the binary value is determined by the function used.

Part of #5818

This patch introduces mem_set_zerobin() function. This function clears
MEM and sets it to binary value that consists of given number of zero
bytes.

Part of #5818

This patch introduces mem_copy_bin() function. This function copies
given binary value to a newly allocated by MEM memory.

Part of #5818

This patch introduces set of mem_set_bin_*() functions. These functions
clears MEM and sets it to given binary value. Degree of clearing and
type of allocation of the binary value is determined by the function
used.

Part of #5818

This patch introduces mem_copy_str() and mem_copy_str0() functions.
These functions copy given string to newly allocated by MEM memory.
String given to mem_copy_str0() should be NULL-terminated. This is not
necessary for mem_copy_str().

Part of #5818

This patch introduces set of mem_set_str_*() functions. These functions
clears MEM and sets it to given string value. Degree of clearing and
type of allocation of the string is determined by the function used.

Part of #5818

This patch performs a small refactoring of mem_set_double() and moves
this function and mem_set_bool() to another place so that they are part
of the code block containing all mem_set_*() functions.

Part of #5818

This patch introduces mem_set_uint(). This function clears MEM and sets
it to given unsigned value.

Part of #5818

This patch introduces mem_set_int(). This function clears MEM and sets
it to given integer value.

Part of #5818

This patch introduces mem_set_null(). This function clears MEM and sets
it to NULL.

Part of #5818

This patch adds initialization for newly created MEM objects in
sqlVdbeAllocUnpackedRecord(). Changing a MEM without being
initialized may give us unexpected result.

Part of #5818

This patch introduces mem_bit_and(), mem_bit_or(), mem_shift_left(),
mem_shift_right(), and mem_bit_not(), which perform bitwise operations
on MEM.

Part of #5818

This patch introduces set of mem_cmp_*() functions. These functions are
used to compare MEMs.

Part of #5818

This patch introduces mem_add(), mem_sub(), mem_mul(), mem_div() and
mem_rem(), which perform arithmetic operations on two MEMs. Operands
must contain values of numeric types or values that can be converted
to a number according to implicit casting rules.

Part of #5818

This patch introduces mem_concat(). Function mem_concat() concatenates
values from two MEMs in case these values are strings or binaries and
writes the result to the third MEM.

Part of #5818

Function sql_column_to_messagepack() has almost the same functionality
as sql_vdbe_mem_encode_tuple(). Due to this it is not necessary to
have sql_column_to_messagepack(), so it is removed in this commit.

Part of #5818

The original vdbe_decode_msgpack_into_mem() returns a MEM that contains
string and binary values as ephemeral. This patch renames this function
to mem_from_mp_ephemeral() and introduces new function mem_from_mp(),
which returns a MEM that contains string and binary values in newly
allocated memory.

This patch changes behavior for this query:

CREATE TABLE t1(m VARBINARY primary key);
INSERT INTO t1 VALUES(x'6178'), (x'6278'), (x'6379');
SELECT count(*), substr(m,2,1) AS mx FROM t1 GROUP BY mx;

Before this patch:

tarantool> SELECT count(*), substr(m,2,1) AS mx FROM t1 GROUP BY mx;
---
- metadata:
  - name: COLUMN_1
    type: integer
  - name: MX
    type: string
  rows:
  - [2, 'y']
  - [1, 'y']
...

After this patch.

tarantool> SELECT count(*), substr(m,2,1) AS mx FROM t1 GROUP BY mx;
---
- metadata:
  - name: COLUMN_1
    type: integer
  - name: MX
    type: string
  rows:
  - [2, 'x']
  - [1, 'y']
...

Part of #5818
Closes #5890

This patch reworks mem_move(). This function moves all content of source
MEM to destination MEM. Source mem is set to NULL.

Part of #5818

This patch intoduces mem_copy_as_ephemeral(). This function copies value
from source MEM to destination MEM. In case value is of string or binary
type and its allocation type is not static, it copied as value with
ephemeral allocation type.

Part of #5818

This patch introduces mem_copy(). This function copies value from source
MEM to destination MEM. In case value is string or binary and have not
static allocation type, it is copied to newly allocated memory.

Part of #5818

This patch introduces mem_is_*() functions that allows to check current
MEM state.

Part of #5818

This patch introduces mem_destroy(). This function should be used to
free and destroy all objects owned by MEM, if necessary.

Part of #5818

This patch introduces mem_create(). This function should be used to
initialize a MEM. MEM should be initialized before usage.

Part of #5818

This patch introduces mem_str() which allows to receive value of MEM as
a string. Due to the limitations of static_alloc(), this function cannot
be used to safely retrieve a value of MEM converted to string. This
function is suitable for debugging, displaying the value in an error
message, etc.

Part of #5818

This patch disables unused code in sql/vdbemem.c. It will simplify
refactoring.

Part of #5818

Part of #5818

This patch simplifies port_vdbemem_dump_lua() and
port_vdbemem_get_msgpack() functions.

Part of #5818

This patch moves all MEM-related functions in mem.c/mem.h.

Part of #5818

There is no need for NULL-termination, since MEMs converted to msgpack
and msgpack do not require NULL-terminated strings.

Needed for #5818

This patch disables unused code in sql/legacy.c. It will simplify
refactoring.

Needed for #5818