Use strlen() 3 times instead of custom length check implementations in ustring

[Ivorforce]

strlen(char32_t *) unfortunately does not exist and has to be added. The implementation speed should be the same or slightly better, except for the char *p_str case where it may be slightly better now, because the compiler might not previously have realized that it can use the optimized strlen instead of rolling its own loop.

[Ivorforce]

Here is a demonstration of the difference in generated bytecode between the two approaches:

https://godbolt.org/z/Tjhs9hWr7 (-O2: https://godbolt.org/z/eGaEKj3oY)

(the function will likely be inlined since it's defined in the same file)

It's unlikely to make a noticeable difference; it's better to view the PR as a clean-up refactor, but it is slightly better.

[adamscott]

Seems ok! I would love a review from @KoBeWi or @Calinou too before merging.

[Calinou]

Code looks good to me.

[bruvzg]

This should not make any noticeable difference, code looks fine.

[Repiteo]

Thanks!