refactor: define a UINT256_ONE global constant

Instead of having a uint256 representations of one scattered throughout
where it is used, define it globally in uint256.h

src/script/interpreter.cpp

@@ -1281,13 +1281,11 @@ uint256 SignatureHash(const CScript& scriptCode, const T& txTo, unsigned int nIn
         return ss.GetHash();
     }
 
-    static const uint256 one(uint256S("0000000000000000000000000000000000000000000000000000000000000001"));
-
     // Check for invalid use of SIGHASH_SINGLE
     if ((nHashType & 0x1f) == SIGHASH_SINGLE) {
         if (nIn >= txTo.vout.size()) {
             //  nOut out of range
-            return one;
+            return UINT256_ONE();
         }
     }