py: Tidy up variables in VM, probably fixes subtle bugs.
Things get tricky when using the nlr code to catch exceptions. Need to ensure that the variables (stack layout) in the exception handler are the same as in the bit protected by the exception handler. Prior to this patch there were a few bugs. 1) The constant mp_const_MemoryError_obj was being preloaded to a specific location on the stack at the start of the function. But this location on the stack was being overwritten in the opcode loop (since it didn't think that variable would ever be referenced again), and so when an exception occurred, the variable holding the address of MemoryError was corrupt. 2) The FOR_ITER opcode detection in the exception handler used sp, which may or may not contain the right value coming out of the main opcode loop. With this patch there is a clear separation of variables used in the opcode loop and in the exception handler (should fix issue (2) above). Furthermore, nlr_raise is no longer used in the opcode loop. Instead, it jumps directly into the exception handler. This tells the C compiler more about the possible code flow, and means that it should have the same stack layout for the exception handler. This should fix issue (1) above. Indeed, the generated (ARM) assembler has been checked explicitly, and with 'goto exception_handler', the problem with &MemoryError is fixed. This may now fix problems with rge-sm, and probably many other subtle bugs yet to show themselves. Incidentally, rge-sm now passes on pyboard (with a reduced range of integration)! Main lesson: nlr is tricky. Don't use nlr_push unless you know what you are doing! Luckily, it's not used in many places. Using nlr_raise/jump is fine.