Parse MSB and LSB from pitch bend and song position messages when using midilib gem

[mike-bourgeous]

This change computes the full value parameter using both MSB and LSB for the MIDI::PitchBend and MIDI::SongPointer message types from the midilib gem. I updated the specs to match.

I used multiplication and addition rather than bitwise operations (shift and or) because * and + benchmarked much faster than << and |.

Thanks for writing nibbler!

[mike-bourgeous]

I also verified this change produces the right output in my own code:

Before:

After: