Add LIKELY and UNLIKELY hints

Doesn’t seem to affect timings in perftest on my machine, but it may
help others.

include/rapidjson/reader.h

@@ -530,15 +530,15 @@ class GenericReader {
      */
     template <unsigned parseFlags, typename InputStream, typename Handler>
     bool IterativeParseNext(InputStream& is, Handler& handler) {
-        while (is.Peek() != '\0') {
+        while (RAPIDJSON_LIKELY(is.Peek() != '\0')) {
             SkipWhitespaceAndComments<parseFlags>(is);
 
             Token t = Tokenize(is.Peek());
             IterativeParsingState n = Predict(state_, t);
             IterativeParsingState d = Transit<parseFlags>(state_, t, n, is, handler);
 
             // If we've finished or hit an error...
-            if (IsIterativeParsingCompleteState(d)) {
+            if (RAPIDJSON_UNLIKELY(IsIterativeParsingCompleteState(d))) {
                 // Report errors.
                 if (d == IterativeParsingErrorState) {
                     HandleError(state_, is);