First, get the body data from the MultiPatch that is located at the given path. Then, process that data through the supplied SinglePatchTruss ToMidiFn.
Pass the body data of the MultiPatch through each of the passed ToMidiFn's, and concatenate the results of all of them.
Returns a MidiMessage that is just the single byte value. TODO: is this used?
The function should have the signature ([[SynthPath:Byte]], Editor) => [MidiMessage]