This patch introduces new rules to determine type of CASE operation.

Part of #6990

@TarantoolBot document
Title: New rules to determine type of result of CASE

New rules are applied to determine the type of the CASE operation. If
all values are NULL with no type, or if a bind variable exists among
the possible results, then the type of CASE is ANY. Otherwise, all NULL
values with no type are ignored, and the type of CASE is determined
using the following rules:
1) if all values of the same type, then type of CASE is this type;
2) otherwise, if any of the possible results is of one of the
incomparable types, then the type of CASE is ANY;
3) otherwise, if any of the possible results is of one of the
non-numeric types, then the type of CASE is SCALAR;
4) otherwise, if any of the possible results is of type NUMBER, then the
type of CASE is NUMBER;
5) otherwise, if any of the possible results is of type DECIMAL, then
the type of CASE is DECIMAL;
6) otherwise, if any of the possible results is of type DOUBLE, then the
type of CASE is DOUBLE;
7) otherwise the type of CASE is INTEGER.