Revive type class elaboration

[facundominguez]

We discovered recently that code to reason about type classes is not tested in the test suite. There are tests at hand that could be used for that sake, with the caveat that they likely will fail on the bit-rotted implementation that we have at the moment.

A project idea has spontaneously materialized, then, where these tests could guide fixing of the code. And then we could select some relevant fragments from them to include in the LH testsuite.