LibCrypto: Implement HMAC

Author: https://github.com/alimpfard
Commit: f2cd004d11
Pull-request: https://github.com/SerenityOS/serenity/pull/1661
Reviewed-by: https://github.com/Dexesttp
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/itamar8910

Libraries/LibCrypto/Cipher/Mode/Mode.h

@@ -50,6 +50,8 @@ namespace Cipher {
                 return ByteBuffer::create_uninitialized(input_size + T::block_size() - remainder);
         }
 
+        virtual String class_name() const = 0;
+
     protected:
         T& cipher() { return m_cipher; }