Generators have a "gi_running" attribute (coroutines have an equivalent cr_running flag). Internally frame also has an executing flag.

We use these, plus the test `f_stacktop pointer == NULL`, to determine what state a generator or frame is in.

It would be much cleaner to maintain a single f_state field in the frame to track the state of a frame, reducing the change of ambiguity and error.

The possible states of a frame are:
CREATED -- Frame exists but has not been executed at all
SUSPENDED -- Frame has been executed, and has been suspended by a yield
EXECUTING -- Frame is being executed
RETURNED -- Frame has completed by a RETURN_VALUE instruction
RAISED -- Frame has completed as a result of an exception being raised
CLEARED -- Frame has been cleared, either by explicit call to clear or by the GC.