The MP_OBJ_STOP_ITERATION optimisation is a shortcut for creating a
StopIteration() exception object, and means that heap memory does not need
to be allocated for the exception (in cases where it can be used). This
commit allows this optimised object to take an optional argument (before,
it could only have no argument).
The commit also adds some new tests to cover corner cases with
StopIteration and generators that previously did not work.
Signed-off-by: Damien George <damien@micropython.org>
