Add type-indexed type representations (`TypeRep a`)

We have a plan to move to a type-indexed form of TypeRep. This ticket serves to track progress.

The key wiki page is: Typeable.

We would like to invite comments and discussion from the community using this ticket as a more permanent home than email.

There is a also a broader question, about using the new expressiveness of TypeRep and Typeable to support static pointers. One particular point that would benefit from more eyes is the polymorphic static pointers support. But first things first!