Add USB Midi support.

It also introduces a txbuffer which copies data into it but passes
the buffer straight to the USB rather than another copy.

src/tusb.h

@@ -89,6 +89,10 @@
     #include "class/msc/msc_device.h"
   #endif
 
+  #if CFG_TUD_MIDI
+    #include "class/midi/midi_device.h"
+  #endif
+
   #if CFG_TUD_CUSTOM_CLASS
     #include "class/custom/custom_device.h"
   #endif
@@ -128,4 +132,3 @@ static inline void tusb_task(void)
 #endif
 
 #endif /* _TUSB_H_ */
-