[slozier]

Fixes exception context chaining so test_contextlib no longer hangs (no __context__ self-reference or cycles). Additional fixes: raise e keeps the existing __cause__, gen.throw() doesn't chain onto the exception being handled, and a bare raise with no active exception raises RuntimeError. Also contains a bit of code cleanup (and some trimmed trailing whitespace that my editor really wanted gone).

Enables test_contextlib (fully passing on 3.4) and test_raise.