JSON: Improve json::decode() performance

- Improves "load" time by ~28% across all performance tests.
- Improves the "add" performance test by ~20% since "load" takes most
  of the time there.

src/JSON.cpp

@@ -405,12 +405,14 @@ std::string json::encode (const std::string& input)
 std::string json::decode (const std::string& input)
 {
   std::string output;
-  for (unsigned int i = 0; i < input.length (); ++i)
+  size_t pos = 0;
+
+  while (pos < input.length ())
   {
-    if (input[i] == '\\')
+    if (input[pos] == '\\')
     {
-      ++i;
+      ++pos;
-      switch (input[i])
+      switch (input[pos])
       {
         // Simple translations.
         case '"':  output += '"';  break;
@@ -424,19 +426,24 @@ std::string json::decode (const std::string& input)
 
         // Compose a UTF8 unicode character.
         case 'u':
-        output += utf8_character (utf8_codepoint (input.substr (++i)));
+          output += utf8_character (utf8_codepoint (input.substr (++pos)));
-        i += 3;
+          pos += 3;
           break;
 
         // If it is an unrecognized sequence, do nothing.
         default:
           output += '\\';
-        output += input[i];
+          output += input[pos];
           break;
       }
+      ++pos;
     }
     else
-      output += input[i];
+    {
+      size_t next_backslash = input.find ('\\', pos);
+      output.append (input, pos, next_backslash - pos);
+      pos = next_backslash;
+    }
   }
 
   return output;