toUTFz again

[Andrej Mitrovic]

I've completely lost track of what happened with the whole toUTF16z
story, but anyway since it's in std.utf why doesn't it just forward to
toUTFz?

const(wchar)* toUTF16z(T)(T input)
    if (isSomeString!T)
{
    return toUTFz!(const(wchar)*)(input);
}

That way it can take any string argument and not just a UTF8 string.
Currently it only accepts UTF8 which is an unnecessary restriction.

Secondly, it's difficult to make an alias to 'toUTFz'. For example, if
you want a short version of 'toUTFz!(char*)' in your code, you would
typically write an alias like this:

alias toUTFz!(char*) toCharPtr;

However that won't work because toUTFz requires a second type
argument. If toUTFz was a template that forwarded to other
implementation templates it would make the above alias possible.
Here's what I mean:

// equivalent to one of toUTFz templates in std.utf
auto toUTFzImpl(P, S)(S s) { return null; }

template toUTFz(P)
{
    P toUTFz(S)(S str)
    {
        return toUTFzImpl!(P)(str);
    }
}

void main()
{
    alias toUTFz!(char*) toUTF8z;
    toUTF8z("foo");
    toUTF8z("foo"w);
    toUTF8z("foo"d);
}

That's much simpler than having to declare every possible combination
just to use a simple alias:
alias toUTFz!(const(char*), char[]) toUTF8z;
alias toUTFz!(const(char*), wchar[]) toUTF8z;
alias toUTFz!(const(char*), dchar[]) toUTF8z;
alias toUTFz!(const(char*), string) toUTF8z;
alias toUTFz!(const(char*), wstring) toUTF8z;
alias toUTFz!(const(char*), dstring) toUTF8z;