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: