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

@@ -214,6 +214,11 @@ 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
     {

src/serialize.h

@@ -156,6 +156,8 @@ inline float ser_uint32_to_float(uint32_t y)
 // i.e. anything that supports .read(char*, size_t) and .write(char*, size_t)
 //
 
+class CSizeComputer;
+
 enum
 {
     // primary actions
@@ -230,6 +232,8 @@ inline unsigned int GetSizeOfCompactSize(uint64_t nSize)
     else                            return 9;
 }
 
+inline void WriteCompactSize(CSizeComputer& os, uint64_t nSize);
+
 template<typename Stream>
 void WriteCompactSize(Stream& os, uint64_t nSize)
 {
@@ -323,6 +327,9 @@ inline unsigned int GetSizeOfVarInt(I n)
     return nRet;
 }
 
+template<typename I>
+inline void WriteVarInt(CSizeComputer& os, I n);
+
 template<typename Stream, typename I>
 void WriteVarInt(Stream& os, I n)
 {
@@ -906,6 +913,17 @@ inline void SerReadWrite(Stream& s, T& obj, CSerActionUnserialize ser_action)
 
 
 
+/* ::GetSerializeSize implementations
+ *
+ * Computing the serialized size of objects is done through a special stream
+ * object of type CSizeComputer, which only records the number of bytes written
+ * to it.
+ *
+ * If your Serialize or SerializationOp method has non-trivial overhead for
+ * serialization, it may be worthwhile to implement a specialized version for
+ * CSizeComputer, which uses the s.seek() method to record bytes that would
+ * be written instead.
+ */
 class CSizeComputer
 {
 protected:
@@ -921,6 +939,12 @@ public:
         this->nSize += _nSize;
     }
 
+    /** Pretend _nSize bytes are written, without specifying them. */
+    void seek(size_t _nSize)
+    {
+        this->nSize += _nSize;
+    }
+
     template<typename T>
     CSizeComputer& operator<<(const T& obj)
     {
@@ -984,6 +1008,17 @@ inline void SerReadWriteMany(Stream& s, CSerActionUnserialize ser_action, Args&.
     ::UnserializeMany(s, args...);
 }
 
+template<typename I>
+inline void WriteVarInt(CSizeComputer &s, I n)
+{
+    s.seek(GetSizeOfVarInt<I>(n));
+}
+
+inline void WriteCompactSize(CSizeComputer &s, uint64_t nSize)
+{
+    s.seek(GetSizeOfCompactSize(nSize));
+}
+
 template <typename T>
 size_t GetSerializeSize(const T& t, int nType, int nVersion = 0)
 {