Add support for CREATE TABLE AS statements

[Just-Zach]

Fixes #175

Adds support for CREATE TABLE ... AS SELECT as defined in https://www.postgresql.org/docs/current/sql-createtableas.html

Implementation was kept narrow to the standard CREATE TABLE AS form:

 CREATE TABLE [IF NOT EXISTS] table_name [(column_name [, ...])]
      AS query
      [ WITH [ NO ] DATA ]

Example query:

CREATE TABLE foo AS SELECT * FROM bar

• New tests added
• Existing tests pass