Classes: test that classes can implement parameterized ifaces

[catamorphism]

This probably doesn't work right now.

[catamorphism]

Partially addressed in 4f105e4 but the test case in 1e6da7b should pass in order for this to be considered done. #2364 blocks this.

[nikomatsakis]

I found this wasn't really working at all. More-or-less fixed in a3be0b1, probably bugs still remain. @catamorphism I am 99% sure what I did was right, but lemme know if you disagree.

[catamorphism]

This seems to work. Added a test case, closing; open new bugs if there are issues.