py: Add mp_pending_exception global variable, for VM soft interrupt.
This allows to implement KeyboardInterrupt on unix, and a much safer ctrl-C in stmhal port. First ctrl-C is a soft one, with hope that VM will notice it; second ctrl-C is a hard one that kills anything (for both unix and stmhal). One needs to check for a pending exception in the VM only for jump opcodes. Others can't produce an infinite loop (infinite recursion is caught by stack check).