Migration grouping helpers

Ability to add and remove GROUPING FOREIGN KEYS during migration via new helper methods (see README or grouping_test for examples).

I debated something like `group_to` for the table attached methods but couldn't settle on a better verb than 'grouping'. Easy to change so please suggest something if it seems better.

Note that this *does not* manage the reference column. I considered automatically calling those if the column wasn't present but opted for less magic. We could have e.g. `grouping_reference` that does both, too. Please chime in if either of those seems useful.

This type of addition isn't directly supported, hence some of the monkey patching on Table and SchemaDumper and duplication in SchemaCreation. Looks like they might be improving the situation in 4.1, for what that's worth.