Add support for GHC 7.10 `integer-gmp2` & `Natural`

This adds support for the new `integer-gmp2` `Integer`-backend
available with GHC 7.10, which has a different internal
representation. The instance is written in such a way to
have the same hash-mapping as with the old `integer-gmp` backend.

Moreover, this also adds an instance for GHC 7.10's new `Natural`
type.