Allow structs to be parsed in JSON with out of order fields.

Also simplified the code and made it faster.

Change-Id: I1d83b1165a4a9a4380d1bfb5538769c012d2d367
Tested: on Linux.

tests/MyGame/Example/Monster.py

@@ -4,6 +4,7 @@
 
 import flatbuffers
 
+# /// an example documentation comment: monster object
 class Monster(object):
     __slots__ = ['_tab']