std.fmt.hex: support integer sizes that are not powers of two

[myclevorname]

Fixes #23799

This changes the return type, but I don't know of any targets where this is actually breaking change.

[RetroDev256]

Would we also want to support integer sizes that are not a multiple of 8? Not exactly saying we should, but I am wondering if there is merit to making this work for all unsigned integers.

[myclevorname]

Would we also want to support integer sizes that are not a multiple of 8? Not exactly saying we should, but I am wondering if there is merit to making this work for all unsigned integers.

That's a good idea. Let me work on that.

[wooster0]

While you're at it do you mind adding a case: Case parameter for allowing uppercase too? It's not difficult to support.

[myclevorname]

While you're at it do you mind adding a case: Case parameter for allowing uppercase too? It's not difficult to support.

My idea was to introduce a hexOptions function with options for that, and then reimplementing hex on that. I am balancing school, life, and hobbies, so this won't get done very quickly.

[wooster0]

Not sure which option this function would ever need beside case. Also I'm guessing that would be biased towards lowercase as the default which I can't say I'm a fan of. It's fine to break code if you're worried about that.