Add conformance tests for `Generic.FunctionField` / fix bugs this reveals

[fingolfin]

Actually I add some in #1686 but commented them out as they currently don't terminate, hinting at a bug in in the function field code.

[lgoettgens]

It seems to me that the random elements that test_elem(R) produces here are just way too big to be handled efficiently. Even calculating a^4 for such a random element takes too much time