mbv: now with functioning timer interrupts!

And a few other nice things.
The bootloader now has an embedded wozmon!
If you know its offset, you can jump to it from the app.

mbv/hal/interrupts.h

@@ -0,0 +1,6 @@
+#pragma once
+
+using Isr = void(*)();
+
+void SetExternalInterruptHandler(Isr handler);
+void EnableExternalInterrupts();