refactor: std::string -> std::string_view

Safer after recent fix to make TxidFromString() respect string_view length. The internal call to uint256S() when parsing the txid would still have stopped at the '-'-symbol in this case though.

src/rest.cpp

@@ -789,8 +789,9 @@ static bool rest_getutxos(const std::any& context, HTTPRequest* req, const std::
         for (size_t i = (fCheckMemPool) ? 1 : 0; i < uriParts.size(); i++)
         {
             int32_t nOutput;
-            std::string strTxid = uriParts[i].substr(0, uriParts[i].find('-'));
-            std::string strOutput = uriParts[i].substr(uriParts[i].find('-')+1);
+            const std::string_view part{uriParts[i]};
+            const std::string_view strTxid{part.substr(0, part.find('-'))};
+            const std::string_view strOutput{part.substr(strTxid.size()+1)};
 
             if (!ParseInt32(strOutput, &nOutput) || !IsHex(strTxid))
                 return RESTERR(req, HTTP_BAD_REQUEST, "Parse error");