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, `holes` and `transformChildren`, in all their variants, seem to be non-recursive. I notice I don't have a test that demonstrates that for `holes`.

From your first comment, it sounds like `transformChildren2` should do the same transformation as `transformChildren`, which means it's broken and I'll take a look at it.