[sdiy] MIDI bytestream parser

[Olivier Gillet]

> What's all this 'real' and 'encoded' stuff? Is that what the hash is for? What are you up to?

I think it's just shifting the channel number from 1-16 (what's
exposed to the user) to 0-15 values (what's used internally by MIDI).

The HASH( ) is used to implement a lookup map from status byte to
handler function, by observing that there are no two status byte with
the same nibble checksum (they are either nx, with n between 8 and E
and x for channel messages, or Fn, with n below 8 for system messages.