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/intc.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <cstdint>
+
+using Isr = void(*)(void);
+
+/// Returns: true if the IRQ was previously enabled
+bool SetIrqEnabled(uint8_t irqn, bool enabled);
+
+void SetIsr(uint8_t irqn, Isr isr);
+
+// Call this once to enable all HW interrupts
+void EnableInterrupts();
+
+// Feed this to the CPU's interrupt handler
+void InterruptHandler();