optimize general sorted(x) calls

Picking up where I left off in gh-343, here's a patch to generate custom code for all calls of the form `sorted(x)`.

I'm not sure if I understood the meaning of `is_temp` correctly in the `PySequence_List` function type. Hardcoding it to `True` doesn't give different code in my test examples.

It might be an idea to generate `PySequence_List` for some or all calls to `list`; it apparently catches non-sequence inputs exactly as the `list` constructor in pure Python does.
Oh, and this assertion was actually true even in `master`, so I turned it back on.