loxi-prep: remove uses of cstruct module

This module does not exist in PyLoxi.
loxi-prep: access message/action modules through the ofp module
loxi-prep: remove action_ prefix from callers
loxi-prep: rename action_list.add to append

For consistency with the list type.
action_list: remove unused methods
action_list: remove show method

Also adds __iter__ so we can use an action_list as a sequence.
loxi-prep: use plain lists to hold OF actions

Some callsites needed to be changed because they depended on action_list.append
taking a shallow copy.
loxi-prep: rename match fields to match pyloxi
loxi-prep: rename uses of match fields
loxi-prep: rename stats reply 'stats' field to 'entries' to match PyLoxi
loxi-prep: rename error 'type' field to 'err_type'
loxi-prep: rename stats request/type field 'type' to 'stats_type'
loxi-prep: move header fields into the message classes
loxi-prep: default xid to None
loxi-prep: subtype flow_mod classes
loxi-prep: move parse.py to oftest module
loxi-prep: move message parsing to ofp.message
loxi-prep: change parse_header to return a tuple
loxi-prep: remove import compatibility
loxi-prep: always handle missing entry in type map