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.cpp

@@ -204,6 +204,23 @@ unsigned int CScript::GetSigOpCount(const CScript& scriptSig) const
     return subscript.GetSigOpCount(true);
 }
 
+bool CScript::IsPayToAnchor() const
+{
+    return (this->size() == 4 &&
+        (*this)[0] == OP_1 &&
+        (*this)[1] == 0x02 &&
+        (*this)[2] == 0x4e &&
+        (*this)[3] == 0x73);
+}
+
+bool CScript::IsPayToAnchor(int version, const std::vector<unsigned char>& program)
+{
+    return version == 1 &&
+        program.size() == 2 &&
+        program[0] == 0x4e &&
+        program[1] == 0x73;
+}
+
 bool CScript::IsPayToScriptHash() const
 {
     // Extra-fast test for pay-to-script-hash CScripts:

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;

src/script/sign.cpp

@@ -475,6 +475,9 @@ static bool SignStep(const SigningProvider& provider, const BaseSignatureCreator
 
     case TxoutType::WITNESS_V1_TAPROOT:
         return SignTaproot(provider, creator, WitnessV1Taproot(XOnlyPubKey{vSolutions[0]}), sigdata, ret);
+
+    case TxoutType::ANCHOR:
+        return true;
     } // no default case, so the compiler can warn about missing cases
     assert(false);
 }

src/script/solver.cpp

@@ -24,6 +24,7 @@ std::string GetTxnOutputType(TxoutType t)
     case TxoutType::SCRIPTHASH: return "scripthash";
     case TxoutType::MULTISIG: return "multisig";
     case TxoutType::NULL_DATA: return "nulldata";
+    case TxoutType::ANCHOR: return "anchor";
     case TxoutType::WITNESS_V0_KEYHASH: return "witness_v0_keyhash";
     case TxoutType::WITNESS_V0_SCRIPTHASH: return "witness_v0_scripthash";
     case TxoutType::WITNESS_V1_TAPROOT: return "witness_v1_taproot";
@@ -165,6 +166,9 @@ TxoutType Solver(const CScript& scriptPubKey, std::vector<std::vector<unsigned c
             vSolutionsRet.push_back(std::move(witnessprogram));
             return TxoutType::WITNESS_V1_TAPROOT;
         }
+        if (scriptPubKey.IsPayToAnchor()) {
+            return TxoutType::ANCHOR;
+        }
         if (witnessversion != 0) {
             vSolutionsRet.push_back(std::vector<unsigned char>{(unsigned char)witnessversion});
             vSolutionsRet.push_back(std::move(witnessprogram));

src/script/solver.h

@@ -22,6 +22,7 @@ template <typename C> class Span;
 enum class TxoutType {
     NONSTANDARD,
     // 'standard' transaction types:
+    ANCHOR, //!< anyone can spend script
     PUBKEY,
     PUBKEYHASH,
     SCRIPTHASH,