Add experimental player key buffering.

[AliceLR]

When a player input key is pressed and released on the same cycle, the player (and IF UPPRESSED, etc.) will now detect that as a key press. This helps a lot for playing at very low MZX speeds.

• TODO: it may be desirable to have this affect all calls of get_key_status instead of only for keycode_internal_wrt_numlock, as this would mean that the KEYn counters (which use keycode_pc_xt) would also benefit from this. Further feedback is required. Implemented.