Avoid double negation in Emitter.prototype.jumpIfNot.
Move Context.prototype.keys to wrapGenerator.keys.

This function doesn't need to be a method of the Context, and might be
useful externally. More importantly, it's a similar function to
wrapGenerator.values, which will definitely need to be available
externally (to support for-of loops outside of generator functions).
Support for-of iteration over iterables, generators, and Arrays.

Closes #27.
Allow yield* to accept any iterable argument.

Closes #108.