[Proposal] Add module for C-strings support in Phobos

["Nordlöw"]

> #!/usr/bin/env rdmd-dev-module
>
> unittest {
>     import std.stdio: wln = writeln;
>     import std.string;
>     wln(typeof("a".z).stringof);
> }

Correction:

unittest {
     import std.stdio: wln = writeln;
     import std.string;
     alias z = toStringz;
     wln(typeof("a".z).stringof);
}

gives same error

t_string.d(7,19): Error: no property 'z' for type 'string'