Locally install the library, for doctests

"cabal-dev build" was not sufficient for the doctest tool to resolve
demo's import of "Control.Commutative".
"cabal-dev install" does the trick, and only installs to "./cabal-dev".
Remove Unordered_Nat

Using Int as Nat was really not appropriate this time.
Nat

The traditional inefficient unary representation for natural numbers.
I want to implement the intrinsically-commutative versions of add and
mul from my thesis.
QuickCheck for Nat

Test by generating instances of Nat, not instances of Int. Otherwise a
lot of the tests are wasted because negative instances are ignored.