much improved coverage and a few minor fixes in fixplate

Including coverage for essentially everything except for the morphisms
and a few odds and ends.

Fixes a handful of methods which did not work as advertised, and a couple of
type class instances which didn't seem to be usable as typed.

Note: a couple of tests are failing because I could not determine what the correct result should be.
Yes, that's the idea. Here's the Haskell version:
http://hackage.haskell.org/package/fixplate-0.1.5/docs/src/Data-Generics-Fixplate-Open.html#project