Proposal: Migrate Data.Functor.{Sum, Product, Compose} from transformers to base.

[Ryan Scott]

I really like this proposal, especially since having convenient Eq1,
Ord1, Read1, Show1, etc. instances is something I've wanted for a
while.

I have some questions about some specifics:

1. Just to verify, you're proposing a breaking change in which
Data.Functor.Constant no longer exports Constant, but rather Const?
2. You mentioned that the additions to base could use
DefaultSignatures. Could you give an example of how this would work?