- rework MySQL + autogenerate so that

- rework MySQL + autogenerate so that
multiple changes to a single col are collapsed
into one step, will work for other dialects
which may support this too
- add support for "imports" in scripts so
that dialect-specific types can be rendered straight in
from their parent module and work immediately
- rework the internals of autogenerate to be more
succinct, though there's a lot more that could
happen here to make this easier