Skip over phantom type parameters when generating instance context.

A parameter is considered phantom if it does not appear in the body of
a datatype or newtype definition. This is a conservative approximation,
because in general field types could themselves mention parameters that
are actually phantom.

Fixes #13
Update test to include a phantom type param.
Move source repository location to top.
Make t/Test.hs a test-suite in the .cabal file.