* Change node_map type from map<ptr,ptr> to vector<pair<ptr,ptr>>

Map nodes are now iterated over in document order.

* Change insert_map_pair to always append

Always append in insert_map_pair even if the key is already present.
This breaks the behavior of force_insert which now always inserts KVs
even if the key is already present. The first insert for duplicated keys
now takes precedence for lookups.