WString: return bool instead of unsigned char

[mcspr]

Clean up the intent, resulting assembly stays the same as far as I could tell (objdump + vimdiff)
Follow-up #7781

if(s) helper is kind of redundant though, since c_str()buffer() may never be nullptr and always points to something. Still needed to be compatible though, if something uses it

[earlephilhower]

Thanks, I really like the idea, @mcspr !

That said, I'm worried about changing the signature of something as basic as String. Even today, in the ArduinoAPI repo String.h uses unsigned char. So this change would move our interface away from that, and break anything inheriting from String.
https://github.com/earlephilhower/ArduinoCore-API/blob/9da53735171f7cccbe781c66d9ee1068d94d6ec8/api/String.h

So I'm honestly not sure which way to go here. Let's discuss and see if we can get anyone else's input.

[mcspr]

I may be missing the actual issue, but since this is not virtual inheritance methods do not depend on the base signature:
https://godbolt.org/z/x9b5be8b4
Removing the implementation will use the BaseX method implicitly, but if method signature is actually different one needs to do a using BaseX::method; to make it use the base method.

Also opened a PR to the ArduinoCore-API with a similar change

[earlephilhower]

Solid explanation. I'm happy to give this a shot and see if we uncover some weirdness in ancient libs, since it looks like the odds are very low.