This adds MIDI support to the language as a new unstable feature. There
are two new metacommands that come with this:
$Unstable: Midi
$MidiSoundFont: [Default|"filename"]
The $Unstable command is required to be able to use any of the other
commands, and just signifies that this is not a full part of the
language yet and may change in breaking ways before the API is
finalized.
The $MidiSoundFont command enables MIDI support in the compiled program,
and also specifies what sound font to use to play MIDI files. "Default"
will make use of the soundfont placed at
'./internal/support/default_soundfont.sf2', and otherwise a filename can
be specified to use any soundfont wanted.
In either case, the selected sound font is compiled into the executable
and then loaded at runtime.
Fixes: #115
