Quickcheck

Have some quickcheck tests. :)

- test cases are in elm/tests/Main.hs
- there a bunch of newtypes of Strings with Arbitrary instances in elm/tests/Util/Arbs.hs

The test cases are using interpolatedstring-perl6 to make it easier to generate parameterized test strings.

Some shell commands:
- elm/test - compile and run tests
- elm/testRepl - open tests in ghci