[basic.fundamental] Use std:: prefix consistently for library type aliases

[eisenwave]

Later in [basic.fundamental], we use std::nullptr_t. To my understanding, we generally want to use std:: when referring to library types from within core wording.

Perhaps there would have been an argument that uint_least16_t is a "C thing" and so unlike std::nullptr_t, it doesn't need a std:: prefix, but now nullptr_t also exists in C23 which defeats any justification for this local inconsistency.

[jwakely]

I don't think the C argument would work here, because char16_t and char32_t are very different in C. It would make little sense to argue for consistency with C while defining things differently from C!