Separate Column Diffs from Table Diffs

[bplunkett-stripe]

All column differs are grouped into table add/alter statement blocks. This makes it difficult to build dependencies with individual columns. If we did this, we would also need to split out check constraints

We might want to reconsider some of the "diffing" paradigm to make this easier, e.g., some sort of support for nested SQLVertexGenerators.

[bplunkett-stripe]

Closing in favor of #131