Support deferred label resolving for immediate values

[ranma]

This may not be the right approach, but seems to work in the simple case of MOV A, #label$L. Would fix: https://github.com/vpelletier/dissn8/issues/4

Note that the opcode sorting is added to avoid running into `if isinstance(right NameError)' too early. A better option would probably be to always try all opcode variants and only raise on NameError later if no matching opcode was found.

DO NOT MERGE