Add optimized CSizeComputer serializers

To get the advantages of faster GetSerializeSize() implementations
back that were removed in "Make GetSerializeSize a wrapper on top of
CSizeComputer", reintroduce them in the few places in the form of a
specialized Serialize() implementation. This actually gets us in a
better state than before, as these even get used when they're invoked
indirectly in the serialization of another object.

src/pubkey.h

@@ -210,6 +210,11 @@ struct CExtPubKey {
     void Decode(const unsigned char code[BIP32_EXTKEY_SIZE]);
     bool Derive(CExtPubKey& out, unsigned int nChild) const;
 
+    void Serialize(CSizeComputer& s) const
+    {
+        // Optimized implementation for ::GetSerializeSize that avoids copying.
+        s.seek(BIP32_EXTKEY_SIZE + 1); // add one byte for the size (compact int)
+    }
     template <typename Stream>
     void Serialize(Stream& s) const
     {