fix basic_string warnings

[oltolm]

This is my attempt to fix the std::basic_string errors when compiling with libc++19. I wasn't sure in some places whether the fix is correct which is why it is a draft. At least it build.

Fixes #16259.

[Nekotekina]

char32_t is not allowed to contain any 32-bit value, it's only for Unicode codepoints.

[elad335]

Replace std::basic_string with std::vector, std::basic_string_view with std::span, avoid changing value types.