Implemented assembly source generation of non-unique local labels.
The new 2023-non-unique-labels test exercises various edge cases
(though we're still missing local variable interaction).
The format of uniquified labels changed slightly, so the expected
output of 2012-label-localizer needed to be updated.
This changes the "no opcode mnemonics" and "mask leading underscores"
functions into integrated parts of the label localization process.
