The code generator(s) for the interpreters and JIT could do with better error reporting and documentation

[markshannon]

Particularly they need:

• Better documentation of DEAD, ERROR_IF, DEOPT_IF and other special macros.
• Checks for use after free. We track liveness of values, so we can report uses of dead variables.

[Lincoln-developer]

hey, @markshannon I would love to work on this if you don't mind, thanks.