transpile: Refactor, document and correct uses of used() flag

[Rua]

• Depends on transpile: Use convert_side_effects_expr in more places #1976

I've documented how is_used is supposed to be set, and added/removed used() accordingly.

If you're wondering what's up with some of the code in the exprs.c test suddenly disappearing: The transpiler normally eliminates unused code, which includes unused results of binary expressions:

• 1 + 1; gets eliminated altogether
• 1 + func(); gets reduced to func();

But for some reason unary expressions were not handled the same way, and were emitted even if they didn't do anything. Now they get removed too.

[fw-immunant]

Code changes all look good, but before merging I want to go verify that this doesn't interfere with the way Hayroll inserts no-op expressions into c2rust inputs to link C preprocessor macro expansions with their c2rust translations.