Support for the CAST SQLite function

[groue]

This pull request adds built-in support for the SQLite CAST function. The cast(_:as:) Swift function accepts expressions, and association aggregates.

cast(Column("wins"), as: .real)
cast(Player.games.count, as: .real)

Fixes #1514

[leotumwattana]

There is a more complex case where using cast will not work.

Please see: #1514 (comment)