Do cpedantic for stdlib & testing

[plajjan]

We should compile stdlib and our test with --cpedantic.

We've concluded in the past that cpedantic can't be enabled per default since it depends on the CC version, and we don't want our use base to be guinea pigs. However, for compiling stdlib and test , i.e. acton itself, we should be fine. Last week I spent a few hours too many on debugging something that would have been caught with --cpedantic.

Outstanding things to fix:

[plajjan]

We now use a particular version of zig so we don't rely on a system CC and thus we can always use --cpedantic.

--cpedantic actually broke a bit with the restructuring to use build.zig and it was never implemeneted for project builds but that is now fixed with #1440 and #1441.

I think we should start to fix these errors, one by one, until they're all fixed and we can permanently enable it. It can be temporarily enabled by like so

CPEDANTIC=1 make

@sydow do you want to start to take a stab at fixing various compilation warnings/errors? :)