Really fix ty_htable_foreach this time

You know, instead of crashing. Don't rush commits, edition #234, I hope
it'll slip in my brain someday.
Fix interface number of MIDI mode
Remove EXPERIMENTAL safeguard for Teensy 3.1
Add ty_board_model_vtable (although empty right now)
Avoid non-Teensy boards in ty_board_identify

Use the vtable to determine if a model is indeed a Teensy one. Testing
non-Teensy models using Teensy-specific fields would be at best just
non-sense, at worst a very intermittent and annoying segmentation fault.