pyloxi: use the ofclass subtemplate for messages

The only difference was the special case for the default XID.
pyloxi: use ofclass subtemplate for OXMs
ir: convert version fields to OFTypeMembers

The frontend gets a new OFVersionMember field type, which is lowered to
OFTypeMember when generating the IR (which is when the version is first known).
pyloxi: use OFClass directly

It now includes everything we need except pyname.
pyloxi: create inheritance hierarchy

Virtual classes are generated but currently only serve as superclasses.

Somehow this works without topologically sorting the classes.
pyloxi: generate virtual class unpack methods

This is done completely from the IR now.
pyloxi: move list unpack functions into util

The goal is to make all module templates identical.
pyloxi: generate all openflow class modules from a single template

common.py and message.py had small amounts of special code left which were
moved into the *_extra.py subtemplates.
pyloxi: remove automatic conversion from str to OFReader

User code shouldn't call these methods directly.
pyloxi: keep track of offset/length ourselves in OFReader

For skip_align to work we need to know the offset from the beginning of the
message, not from the beginning of our OFReader (which might have already been
misaligned).
pyloxi: deal with length fields inside unpack

This will make list parsing much simpler.
pyloxi: remove special case for reading OXM lists

This is now handled automatically by match_v3.unpack.
pyloxi: generic list deserialization

Since objects now handle their own type and length fields they can all use the
same generic list code. The oftype functions now generate list pack, unpack,
and init code on demand so nothing needs to be changed in pyloxi to use a new
list type.
pyloxi: move pack_list to generic_util

For consistency with unpack_list.
ir: add alignment properties to OFClass

Updated pyloxi to use the new properties. Unpack now uses `has_external_alignment`
because internal alignment would be handled by slicing based on the length field.
Merge into master from pull request #147:
Make pyloxi inheritance and list handling generic (https://github.com/floodlight/loxigen/pull/147)