policy: Add OP_1 <0x4e73> as a standard output type

These outputs are called anchors, and allow
key-less anchor spends which are vsize-minimized
versus keyed anchors which require larger outputs
when creating and inputs when spending.

src/script/script.h

@@ -533,6 +533,14 @@ public:
      */
     unsigned int GetSigOpCount(const CScript& scriptSig) const;
 
+    /*
+     * OP_1 <0x4e73>
+     */
+    bool IsPayToAnchor() const;
+    /** Checks if output of IsWitnessProgram comes from a P2A output script
+     */
+    static bool IsPayToAnchor(int version, const std::vector<unsigned char>& program);
+
     bool IsPayToScriptHash() const;
     bool IsPayToWitnessScriptHash() const;
     bool IsWitnessProgram(int& version, std::vector<unsigned char>& program) const;