This changes the internal buffering to the raw 4-byte messages. The
conversion of the messages to a byte-stream moved to the read/write
methods.
It adds a raw packet interface to send and retrieve the raw 4-byte
USB MIDI message:
static inline bool tud_midi_receive (uint8_t packet[4]);
static inline bool tud_midi_send (uint8_t const packet[4]);
MIDI USB packets carry virtual cable/wire/plug data in the packet header,
which cannot be exported in the byte-stream interface. The raw packet
interface allows to send and and receive the complete USB message.
