Mixing default clauses and constraints & columns without types

[prescientmoon]

The two PRs conflicted, so I merged them into one.

• Closes #53 and #40.
• SQLite allows defining columns without a type next to them.

This PR implements support for such syntax under a new dialect flag. This is a breaking change, as the type for ColumnDef changes.

I added a few tests (as in my past PRs), although I'm not sure I added them to the correct file.

I also didn't use a formatter (as I saw you weren't using one either), so feel free to tweak the code if it looks wonky.