Add sanity check after key generation

Add a sanity check to prevent cosmic rays from flipping a bit in the
generated public key, or bugs in the elliptic curve code. This is
simply done by signing a (randomized) message, and verifying the
result.

src/key.h

@@ -136,6 +136,12 @@ public:
     //! Derive BIP32 child key.
     bool Derive(CKey& keyChild, unsigned char ccChild[32], unsigned int nChild, const unsigned char cc[32]) const;
 
+    /**
+     * Verify thoroughly whether a private key and a public key match.
+     * This is done using a different mechanism than just regenerating it.
+     */
+    bool VerifyPubKey(const CPubKey& vchPubKey) const;
+
     //! Load private key and check that public key matches.
     bool Load(CPrivKey& privkey, CPubKey& vchPubKey, bool fSkipCheck);