Adds fromStringz

In this pull request, instead of the currently present special casing in `std.conv.to!string` for `char*`, `std.string.fromStringz` is provided as an alternative solution.

This more readily compliments the existing `std.string.toStringz` function, and allows `to!string` to be consistent with semantics similar to `formattedWrite`/`writeln` etc.

While `std.conv.to!string(const(char*) s)` has not been deprecated, it is open to discussion as to what path would be the best to take if this request is accepted.