rt.util.utf: add attributes

[WalterBright]

Trivial. Just added annotations that the compiler would pass.

[andralex]

Auto-merge toggled on

[schveiguy]

Note that we have to be careful about adding attributes "that pass", especially where @safe is involved. In this case, toUtf16z is returning a pointer to a null-terminated string. While the "trick" passed in this other PR works, it's disingenuous. A @safe function that returns a pointer is only supposed to be returning a pointer to the single element, not an array as toUTF16z is doing.

see this recent discussion: #2053

I feel like toUTF16z was improperly marked @safe, but not sure what we can do about it now.