Remove unnecessary string copy in JSON serialization

Summary:
`asString()` returns a string by value, which means a copy of the string must be made.

We don't actually need to do this at all:
- We know that the dynamic contains a string, thanks to the switch statement
- The function being called with the result, `escapeString`, only wants a StringPiece anyway.

So this copy is a waste. And uses significant CPU in my application.

Reviewed By: ispeters

Differential Revision: D29812197

fbshipit-source-id: 60df668f7501f78f4282717d6896cd891950b6f5

folly/json.cpp

@@ -137,7 +137,7 @@ struct Printer {
         out_ += "null";
         break;
       case dynamic::STRING:
-        escapeString(v.asString(), out_, opts_);
+        escapeString(v.stringPiece(), out_, opts_);
         break;
       case dynamic::OBJECT:
         printObject(v, context);