I haven't made std.conv.to use it yet, and I haven't touched toUTF16z or
toStringz at all, but here's an implementation for toUTFz. After this is
in, we can make std.conv.to use it when converting to character
pointers, and we should probably make it so that we have toStringz,
toWstringz, and toDstringz which use it and return immutable character
pointers and get rid of toUTF16z.

toUTFz no longer guarantees that the string will remain zero-terminated.
If the string can be zero-terminated but isn't immutable and doesn't
need to be copied to have the requested character pointer type, then it
no longer copies. This means that it's possible to have a string which
is zero-terminated and then stops being zero-terminated if you alter the
character one passed its end, but that's not likely to be an issue in
most cases, and a note in the documentation points it out so that
programmers can know about it and deal with it appropriately.

I also put @System on the two overloads of toUTFZ which do pointer
arithmetic. They're obviously @System anyway, but tagging them with it
makes it clearer.