Annotate (comment) `parser.y` and structure it better

[ISSOtm]

src/asm/parser.y is currently very messy, lacking organization and comments. Bison documentation is clear: both /* ... */ and // are supported.

• [ ] Organize rules better, possibly with "section markers" (// ------------- Numeric expressions ------------- for example)
• [ ] Add comments explaining the purpose of rules when relevant, especially when parser conflicts are involved (consider relocexpr_no_str, notably)