py: Check explicitly for memory allocation failure in parser.

Previously, a failed malloc/realloc would throw an exception, which was
not caught.  I think it's better to keep the parser free from NLR
(exception throwing), hence this patch.
py: Add missing dummy functions to pass 1 emitter.
py: Add option to VM to detect stack overflow.
py: Fix VM stack overflow detection.
py: Make sure state/stack of byte code function has at least 1 slot.
py, compiler: Improve stack depth counting.

Much less of a hack now.  Hopefully it's correct!
py: Simplify stack get/set to become stack adjust in emitters.

Can do this now that the stack size calculation is improved.
tests: Add test for multi-comparison.