Implement a debugger

[bikefrivolously]

Further development will easier with the help of a debugger.

• Step through instructions
• Inspect memory
• Insert breakpoints
• Watch memory locations for changes

Currently this seems difficult to implement due to the way instruction decoding and addressing mode is decoded (v, which holds the addressing mode has a short lifetime and inspection by the debugger (which should probably own the CPU) does not seem possible). Some restructuring may be required so this should be branched.