Types with nothing but collections don't work

[JasperHorn]

Inserting a column with default values is a bit weird in sql, and requires a completely separate syntax. However, I think I'm just not going to support this for the moment (there is an easy work-around: just add some field you don't actually use...)

Part of the reason for this is that using UUIDs (#135) would solve the problem as well (because the id field would no longer be using the default value). So, I'm quite possibly going to be able to close this bug at some point without specifically working on it. Still, I'm documenting it as its own issue for the moment.