Compact serialization for scripts

Special serializers for script which detect common cases and encode
them much more efficiently. 3 special cases are defined:
* Pay to pubkey hash (encoded as 21 bytes)
* Pay to script hash (encoded as 21 bytes)
* Pay to pubkey starting with 0x02, 0x03 or 0x04 (encoded as 33 bytes)

Other scripts up to 121 bytes require 1 byte + script length. Above
that, scripts up to 16505 bytes require 2 bytes + script length.

src/main.h

@@ -647,6 +647,20 @@ protected:
     const CTxOut& GetOutputFor(const CTxIn& input, const MapPrevTx& inputs) const;
 };
 
+/** wrapper for CTxOut that provides a more compact serialization */
+class CTxOutCompressor
+{
+private:
+    CTxOut &txout;
+public:
+    CTxOutCompressor(CTxOut &txoutIn) : txout(txoutIn) { }
+
+    IMPLEMENT_SERIALIZE(
+        READWRITE(VARINT(txout.nValue));
+        CScriptCompressor cscript(REF(txout.scriptPubKey));
+        READWRITE(cscript);
+    )
+};