Add companion package for orphan instances.

[kindaro]

See #5.

[kindaro]

Notes:

• I put a tests: True line in cabal.project to make sure that you can type cabal test and have it run out of the box. We can remove this line if you find it unneeded or harmful. See also new-test should imply --enable-tests haskell/cabal#5079.

[robrix]

This seems reasonable enough but I don’t think I follow why it should live in this repository. On a strictly practical note, I’m not sure I want to sign up for maintaining it since I’m not going to use it myself; and it’ll be a lot more convenient for you to maintain if it’s your own repo.

Thoughts?