Support CREATE TABLE AS

[Dandandan]

Currently we don't support CREATE TABLE AS statements.

Also seen here in the table definition (<as subquery clause> )

https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#_11_3_table_definition

This is useful for analytical queries.

[Dandandan]

Implemented in https://github.com/andygrove/sqlparser-rs/pull/206