Would you consider cleaning up gcc warnings with -Wextra and -Wpedantic?

[lukester1975]

https://github.com/lukester1975/stdexec/tree/gcc-warnings if you would and it's any good...

Happy to fix it up as directed if it's useful. There's only a few in the library itself; mostly in the tests.

Thanks

(gcc manual seems wrong r.e. -Wmissing-field-initializers in c++ code: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39589, hence turning that off)

[williamspatrick]

I'd like this. I'm usually finding regressions in unused parameters because we typically compile some code using stdexec with meson warning_level=3.