sql: refactor sql_expr_coll and sql_binary_compare_coll_seq

Lets make sql_expr_coll() return error code. It is needed since we are
going to use this function to detect operands of concatenation with
incompatible collations. Hence, struct coll* has been moved to output
parameters.  For the same reason lets make sql_binary_compare_coll_seq()
return error code instead of struct coll*. The latter is really required
only in one out of six usages, so lets remove it at all.

Needed for #3947