toUTF16 should take buffer as out parameter

`rt.util.utf.toUTF16` takes a static array `buf` and returns its slice (escaping reference to local variable). I changed it to an out parameter, just like [toUTF8](https://github.com/D-Programming-Language/druntime/blob/7a1ca0d74c44c91c887685d6ab57437aa98266a3/src/rt/util/utf.d#L580).