Until now, QCryptographicHash had a big drawback over the underlying C APIs: at least one extra memory allocation. Either you created a QCryptographicHash object (allocates a Private), and enjoy noexcept addData() and resultView(), or you used the static hash() function (which creates a Private on the stack, so doesn't need to allocate it), but then we needed to return in an owning container.

Provide a hashInto() static function that can use the same Private-on-the-stack optimization that hash() has, but allows the user to pass a buffer to write into. This would be noexcept and could match the speed of the underlying C APIs.

Ditto QMessageAuthenticationCode.