changed default size of enums (int) to a smaller type
changed default size of enums (int) to a smaller type

Easily save over 200 bytes for the library by using the smallest type for these MIDI types.
FortySevenEffects/arduino_midi_library,FortySevenEffects/arduino_midi_library,FortySevenEffects/arduino_midi_library,FortySevenEffects/arduino_midi_library