Convert incoming MIDI (and other inputs) to the internal note representation. This has two purposes: note entry, and midi thru.

This module is, in a way, dual to Perform.Midi.Perform. It takes MIDI input to the internal Input representation and back again, while Perform takes the internal representation, in the form of Event, to MIDI output.

The overlapping part is that this module maps Controls to and from MIDI while Play uses Control. They use the same control names, though, so I can reuse code from Control.

One significant difference between Input and MIDI is that MIDI supports two levels of control addressing: note and channel, while Input can only represent note addressing through NoteId. MIDI controls almost all apply at the channel level, but of course these controls all apply at the note level. The result is that a MIDI control that on a keyboard affects the whole channel will only affect the last played note here. When the input is converted back to MIDI it may wind up sharing a channel anyway, at which point the control will go back to being channel global, but if the instrument has multiple channels, I try to distribute between them to keep note_ids on separate channels. This way, multiple channel emitting controls can be mapped to multiple channel using instruments.

This is basically a simplified version of the channel allocation algorithm in Perform.Midi.Perform. It's hard to reuse that algorithm directly because this one has to operate in realtime and can't see which controls the note is going to use.