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.go

@@ -5,6 +5,7 @@ package Example
 import (
 	flatbuffers "github.com/google/flatbuffers/go"
 )
+/// an example documentation comment: monster object
 type Monster struct {
 	_tab flatbuffers.Table
 }