fix pystack exhaust during resume_process_key.

Instead of handling resumed key events in a deep stack, buffer them
until the next main loop iteration. New resume events that may be emitted
during handling of old resumes are prepended to that buffer, i.e. take
precedence over events that happen deeper into the buffer/event stack.
Logical replay order is thus preserved.

tests/keyboard_test.py

@@ -74,6 +74,7 @@ class KeyboardTest:
                 is_pressed = e[1]
                 self.pins[key_pos].value = is_pressed
                 self.do_main_loop()
+        self.keyboard._main_loop()
 
         matching = True
         for i in range(max(len(hid_reports), len(assert_reports))):