Store warning state with some deferred actions

For example, compare:
```asm
opt -Wassert
def x = 0
pusho -Wno-assert
assert warn, x, "🗣️"
popo
; Does not warn.
```
and
```asm
opt -Wassert
pusho -Wno-assert
assert warn, x, "🗣️"
popo
def x = 0
; Warns!
```

(There's also, possibly, an inconsistency between whether `rgbasm -Wno-assert` but `rgblink -Wassert` should warn about those assertion failures.)

I think this issue applies not just to assertions, but generalises to any “deferred” warnings, which can also occur from some link-time expressions. There's no way to silence a specific file's warnings, in particular; we may know that one specific warning is spurious (possibly it's in a dependency we do not wish to patch), but still want to be warned about others.