Use structured bindings in some places:

Most of the new uses either:
* Replace some uses of `tie`
* bind to pairs when iterating through maps

src/ripple/protocol/impl/SField.cpp

@@ -295,10 +295,11 @@ int SField::compare (SField const& f1, SField const& f2)
 SField const&
 SField::getField (std::string const& fieldName)
 {
-    for (auto const & fieldPair : knownCodeToField)
+    for (auto const& [_, f] : knownCodeToField)
     {
-        if (fieldPair.second->fieldName == fieldName)
-            return * (fieldPair.second);
+        (void)_;
+        if (f->fieldName == fieldName)
+            return *f;
     }
     return sfInvalid;
 }

src/ripple/protocol/impl/STParsedJSON.cpp

@@ -477,13 +477,12 @@ static boost::optional<detail::STVar> parseLeaf (
 
         try
         {
-            std::pair<Blob, bool> vBlob (strUnHex (value.asString ()));
+            auto [vBlob, validVBlob] = strUnHex (value.asString ());
 
-            if (! vBlob.second)
+            if (! validVBlob)
                 Throw<std::invalid_argument> ("invalid data");
 
-            ret = detail::make_stvar <STBlob> (field, vBlob.first.data (),
-                                             vBlob.first.size ());
+            ret = detail::make_stvar<STBlob>(field, vBlob.data(), vBlob.size());
         }
         catch (std::exception const&)
         {

src/ripple/protocol/impl/STTx.cpp

@@ -428,9 +428,9 @@ isMemoOkay (STObject const& st, std::string& reason)
             }
 
             // The raw data is stored as hex-octets, which we want to decode.
-            auto data = strUnHex (memoElement.getText ());
+            auto [data, validData] = strUnHex (memoElement.getText ());
 
-            if (!data.second)
+            if (!validData)
             {
                 reason = "The MemoType, MemoData and MemoFormat fields may "
                          "only contain hex-encoded data.";
@@ -459,7 +459,7 @@ isMemoOkay (STObject const& st, std::string& reason)
                 return a;
             }();
 
-            for (auto c : data.first)
+            for (auto c : data)
             {
                 if (!allowedSymbols[c])
                 {