Resolve issue 3166.

These changes decode valid SIGHASH types on signatures in assembly (asm) representations of scriptSig scripts.
This squashed commit incorporates substantial helpful feedback from jtimon, laanwj, and sipa.

src/script/script.h

@@ -601,7 +601,6 @@ public:
         return (size() > 0 && *begin() == OP_RETURN);
     }
 
-    std::string ToString() const;
     void clear()
     {
         // The default std::vector::clear() does not release memory.