Add linkhash accessor functions (lh_table_head(), lh_entry_next(), etc...) to pave the way for making the lh_table and lh_entry structure opaque in the future.

Update the docs to mark all members of those structures deprecated, and
 suggest what to use instead.

json_object_iterator.c

@@ -71,7 +71,7 @@ struct json_object_iterator json_object_iter_begin(struct json_object *obj)
 
 	/// @note For a pair-less Object, head is NULL, which matches our
 	///       definition of the "end" iterator
-	iter.opaque_ = pTable->head;
+	iter.opaque_ = lh_table_head(pTable);
 	return iter;
 }
 
@@ -98,7 +98,7 @@ void json_object_iter_next(struct json_object_iterator *iter)
 	JASSERT(NULL != iter);
 	JASSERT(kObjectEndIterValue != iter->opaque_);
 
-	iter->opaque_ = ((const struct lh_entry *)iter->opaque_)->next;
+	iter->opaque_ = lh_entry_next(((const struct lh_entry *)iter->opaque_));
 }
 
 /**