Course.ListZipper: add findLeft/findRight examples

The current quickcheck properties are not enough to ensure that the
zipper moves when the predicate is false for the current focus but
true elsewhere (in the given direction).

Therefore add a couple of simple examples to cover these cases.
Course.StateT: add quickcheck property on distinct'

Distinct has no examples or properties.  Add a quickcheck property
(copied from Course.State.distinct).
Merge pull request #78 from frasertweedale/more-doctest

More doctest