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.
This looks like "leaf based indexing", which is fine and probably useful -- maybe check to see if it's the same as the Haskell impl???