Supporting `DEFERRABLE` and `INITIALLY DEFERRED` in migrations

[Zach Daniel]

This would be really useful to have in the migration syntax, to avoid needing to hand-write sql. Especially because in postgres there is no `ALTER CONSTRAINT` statement meaning that you have to write the entire constraint creation in raw SQL. Deferrable constraints can greatly simplify the code around working with certain kinds of tree structures and/or bidirectional references, that kind of thing. My current use case for them is to avoid having to figure out the proper order to insert changes made to a tree structure, for both performance and code clarity.

Happy to take a look at doing this, but want to check interest/feasibility/concerns first.