UUID primary keys for ActiveRecord

(Cross post from #7278 <https://github.com/rails/rails/issues/7278>)

Now that ActiveRecord supports uuid datatypes (#6713<https://github.com/rails/rails/issues/6713>)
I suspect that having UUID instead of auto-incrementing primary keys will
become fairly popular. It's already possible to do this yourself but
perhaps it should be built into ActiveRecord?

Just a hunch, but I don't think much needs to be changed in AR. If
we're doing anything in AR to assume that the id is an integer, it's
probably a bug.

The main place where we'll need changes is in migrations and schema.rb
generation.

I'm happy to contribute a patch but was wanting to solicit feedback on
wether this is wanted and if anybody had ideas on how the configuration
should go.

I think the main changes will be in the migrations. Probably need to
somewhat change the api of the create_table method calls. If you can
get something working (even partially) send a link here, or a PR and we
can work on it together.