BIP144: Serialization, hashes, relay (sender side)

Contains refactorings by Eric Lombrozo.
Contains fixup by Nicolas Dorier.
Contains cleanup of CInv::GetCommand by Alex Morcos

src/script/script.h

@@ -643,6 +643,20 @@ public:
     }
 };
 
+struct CScriptWitness
+{
+    // Note that this encodes the data elements being pushed, rather than
+    // encoding them as a CScript that pushes them.
+    std::vector<std::vector<unsigned char> > stack;
+
+    // Some compilers complain without a default constructor
+    CScriptWitness() { }
+
+    bool IsNull() const { return stack.empty(); }
+
+    std::string ToString() const;
+};
+
 class CReserveScript
 {
 public: