Add Json::to_string:

This allows the declaration for FastWriter to be hidden and makes
conversion of Json::Value objects to strings a little less clunky.

src/ripple/app/tx/TransactionEngine.cpp

@@ -92,7 +92,6 @@ TER TransactionEngine::applyTransaction (
         {
             WriteLog (lsFATAL, TransactionEngine) <<
                 "Transaction serdes mismatch";
-            Json::StyledStreamWriter ssw;
             WriteLog (lsINFO, TransactionEngine) << txn.getJson (0);
             WriteLog (lsFATAL, TransactionEngine) << s2.getJson (0);
             assert (false);