v1.5

Improvements over v1.4, largely improvements from testing with RISC OS Pyromaniac tests:

• Negative constants are handled better within expression parser.
• Partial expression parsing is now applied to allow . or {PC} variable handling.
• Better macro argument parsing, avoid erroneous early expression evaluation.
• Exposed --debug options to aid diagnosis of mis-parsing.
• Better (but still not exhaustive) support for | surrounded symbol names.
• Fix for local label handling when forward referencing a label number which already has a back reference.