Give QMessageAuthenticationCode some TLC

QMessageAuthenticationCode is like QCryptographicHash + key. These classes should have identical API, apart from the additional key for QMessageAuthenticationCode.

Over the past few releases, QCryptographicHash has seen many improvements to its API and implementation which have, however, not resulted in equivalent changes to QMessageAuthenticationCode. In no particular order:

• port to QByteArrayView and largely noexcept operations with fixed-size internal buffers
• new OpenSSL3 backend
• addition of move SMFs
• hash() optimization (only create Private, not public class, saving d-pointer allocation)

Implement these for QMessageAuthenticationCode, too.

Already merged is the move of qmessageauthenticationcode.cpp into qcryptographichash.cpp so QMessageAuthenticationCode can use QCryptographicHashPrivate, and the hash() optimization.