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 are failing because I could not determine what the correct result should be.
@jdegoes: This test shows the current result, which doesn't seem useful. I couldn't really tell from the signature what this was supposed to do differently from `transformChildren`, but I don't think this is it.

See also the next test, which shows similar behavior.