Implement warning for unused macro args

[Rangi42]

Fixes #1579

I'm a bit wary of how much new warning output this will create for various projects. But, even those that do -Weverything can always add -Wno-unused-macro-arg if they find the workarounds (e.g. assert isconst("use all args: \#")) to be awkward/annoying.

[ISSOtm]

Hm, let's hope that this doesn't come back to bite us later. I don't see anything wrong, but for some reason I don't feel confident either.

[Rangi42]

No hurry in merging it, then. I'll play around with it some more.

Edit: we should check the behavior of this case:

MACRO foo
  if \1
    \2
  else
    \3
  endc
ENDM

[aaaaaa123456789]

I've seen macros with dummy arguments on purpose. I've written said macros.
I don't mind extra warnings (tools existing beats tools not existing), but this is the kind of thing you'd want off by default. Anyone using -Weverything is asking for ALL THE OUTPUT, so they deserve what they get. For everyone else, this would be a useful thing for projects that can use it, but it shouldn't be turned on by default.

[Rangi42]

Yes, this is enabled only with -Wall.

[aaaaaa123456789]

I'd make it -Wextra, but that's me.

[Rangi42]

Any examples from gbdev chat/projects of mistakes that this would have caught? if not, we're both iffy enough about it that IMO it should probably be closed.

[ISSOtm]

Then let's close it. This has received only criticism.