sql: introduce ADD CONSTRAINT CHECK statement

This patch extends parser's grammar to allow to create CHECK constraints
on already existent tables via SQL facilities.

Closes #3097

@TarantoolBot document
Title: Document ADD CONSTRAINT CHECK statement

Now it is possible to add CHECK constraints to already existent table
via SQL means. To achieve this one must use following syntax:

ALTER TABLE <table> ADD CONSTRAINT <name> CHECK (<expr>);