Add instance for Natural type

[hvr]

The Natural type has been officially added to base-4.8.0.0

[kolmodin]

What differs between Integer and Natural, except that Natural cannot be negative?

data Natural = NatS#                 GmpLimb# -- ^ in @[0, maxBound::Word]@
             | NatJ# {-# UNPACK #-} !BigNat   -- ^ in @]maxBound::Word, +inf[@

We should probably not encode these internal constructors in the encoding format.
In general it looks similar to Integer, so we could use the same approach, or even piggyback on the Integer serialisation.

[hvr]

@kolmodin it's very similiar to Integer, except that it uses Word instead of Int for small values, and that obviously Natural can only represent non-negative numbers...

PS: Please take a look at

• http://hackage.haskell.org/package/integer-gmp-1.0.0.0/candidate/docs/GHC-Integer-GMP-Internals.html

and (for GHC 7.8.*),

• http://hackage.haskell.org/package/integer-gmp-0.5.1.0/docs/GHC-Integer-GMP-Internals.html

for primitives meant to be useful for serialising Integers and Naturals