LibCrypto: Fix issues in the Crypto stack

This commit fixes up the following:
- HMAC should not reuse a single hasher when successively updating
- AES Key should not assume its user key is valid signed char*
- Mode should have a virtual destructor
And adds a RFC5246 padding mode, which is required for TLS

Libraries/LibCrypto/Hash/HashFunction.h

@@ -51,6 +51,8 @@ namespace Hash {
         virtual DigestType peek() = 0;
         virtual DigestType digest() = 0;
 
+        virtual void reset() = 0;
+
         virtual String class_name() const = 0;
     };
 }