Key changes are sometimes not detected

[SteveCooling]

Sometimes a note will hang (not turn off) or not get detected.

This might be caused by bounce in the switches combined with the fact that we send a XOR of the state out of the keyboard reader.

Try to:

• Stop using XOR to get a definitive state of each key.
• Internalize the velocity logic into KeyboardMux - direct access to the key states and easier to work with for the user.

[SteveCooling]

It seems this issue was a race condition in MidiKeys::loop() where PORTB would change over time when it was assumed to be the same. Earlier timing optimizations obviously made this better.

I'm now making a copy of the data on PORTB and using it for the duration of the function call, and I think that solved the issue. Needs more testing though. 🎶