Add generator function support

[searls]

testdouble.js does not currently support generators. Prior to 3.2, td.replace would replace them as it would any other function, but this was accidental in that the resulting functions didn't really behave like generator functions.

We need to think through what a fake generator would look like and how it would be stubbed/verified. Right now I have no real idea because I haven't seen many useful, isolated examples of them.