ustring: Add overloads for const char* fmt . . .
ustring: Add overloads for const char* fmt . . .

. . . in the hope that doing so can help compilers catch some mistakes
that users might make. However, point out they shouldn't rely on that.
We only provide this overload in case it catches such users in the act
of making a mistake, but they should still be careful when writing code.

details:
 (1) This means we can implement sprintf(ustring const&) in terms of
     ustring(char const*) instead of repeating ourselves.
 (2) I provide a 2nd overload of the zero-args case just for symmetry,
     and to avoid creating another ustring for the argument just to copy
     it on return (if not that I expect this to be performance-critical)

https://gitlab.gnome.org/GNOME/glibmm/issues/21#note_537941

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