sql: enforce implicit type conversions

Most DBs (at least PostgreSQL, Oracle and DB2) allow to process
following queries:

CREATE TABLE t1 (id INT PRIMARY KEY);
INSERT INTO t1 VALUES (1.123), ('2');

In this particular case, 1.123 should be simply truncated to 1,
and '2' - converted to literal number 2.

After passing real type to Tarantool (instead of <SCALAR>), example
above would fail without conversions. Thus, lets add implicit
conversions inside VDBE to make this example be legal.
However, still some types conversions must be prohibited. For instance,
<BLOB> can't be converted to integer or floating point numerical,
and vice versa.