py: Merge BINARY_OP_SUBSCR and store_subscr (w/ delete) into subscr.

mp_obj_t->subscr now does load/store/delete.
Merge branch 'master' of github.com:micropython/micropython
py: Add MP_OBJ_STOP_ITERATION and make good use of it.

Also make consistent use of MP_OBJ_NOT_SUPPORTED and MP_OBJ_NULL.
This helps a lot in debugging and understanding of function API.
tests: Move gen_context to import tests, because it relies on import.
py: Rename USE_COMPUTED_GOTOS to USE_COMPUTED_GOTO and enable on stmhal.

On stmhal, computed gotos make the binary about 1k bigger, but makes it
run faster, and we have the room, so why not.  All tests pass on
pyboard using computed gotos.