Implement an unlifted Proxy type, Proxy#

A value of type 'Proxy# a' can only be created through the new,
primitive witness 'proxy# :: Proxy# a' - a Proxy# has no runtime
representation and is thus free.

This lets us clean up the internals of TypeRep, as well as Adam's future
work concerning records (by using a zero-width primitive type.)

Authored-by: Edward Kmett <ekmett@gmail.com>
Authored-by: Austin Seipp <austin@well-typed.com>
Signed-off-by: Austin Seipp <austin@well-typed.com>