lua: add key_def lua module

Needed for #3276.
Fixes #3398.
Fixes #4025.

@TarantoolBot document
Title: lua: key_def module

It is convenient to have tuple compare function into lua-land for the
following case:
 - exporting key from tuple to iterate over secondary non-unique index
   and delete tuples from space
 - support comparing a tuple with a key / a tuple, support merging
   key_defs from Lua
 - factor out key parts parsing code from the tuples merger

A key_def instance has the following methods:
 - :extract_key(tuple)           -> key (as tuple)
    Receives tuple or Lua table. Returns tuple representing extracted
    key.
 - :compare(tuple_a, tuple_b)    -> number
    Receives tuples or Lua tables.
    Returns:
    - a value > 0 when tuple_a > tuple_b,
    - a value == 0 when tuple_a == tuple_b,
    - a value < 0 otherwise.
 - :compare_with_key(tuple, key) -> number
    - a value > 0 when key(tuple) > key,
    - a value == 0 when key(tuple) == key,
    - a value < 0 otherwise.
 - :merge(another_key_def)       -> new key_def instance
    Constructs a new key definition with a set union of key parts
    from first and second key defs
 - :totable()                    -> table
    Dump key_def object as a Lua table (needed to support __serialize
    method)

The root key_def library exports all instance methods directly.

The format of `parts` parameter in the `key_def.new(parts)` call is
compatible with the following structures:
* box.space[...].index[...].parts;
* net_box_conn.space[...].index[...].parts.

Example for extract_key():

```lua
-- Remove values got from a secondary non-unique index.
local key_def_lib = require('key_def')
local s = box.schema.space.create('test')
local pk = s:create_index('pk')
local sk = s:create_index('test', {unique = false, parts = {
    {2, 'number', path = 'a'}, {2, 'number', path = 'b'}}})
s:insert{1, {a = 1, b = 1}}
s:insert{2, {a = 1, b = 2}}
local key_def = key_def_lib.new(pk.parts)
for _, tuple in sk:pairs({1})) do
    local key = key_def:extract_key(tuple)
    pk:delete(key)
end
```