This implements a `Natural` type for representing unsigned arbitrary
precision integers.

When available, `integer-gmp>=1.0.0`'s `BigNat` type is used as
building-block to construct `Natural` as an algebraic data-type.
Otherwise, `Natural` falls back being a `newtype`-wrapper around
`Integer` (as is done in Edward Kmett's `nats` package).

The `GHC.Natural` module exposes an internal GHC-specific API, while
`Numeric.Natural` provides the official & portable API.

Reviewed By: austin, ekmett

Differential Revision: https://phabricator.haskell.org/D473