loci: remove dead offset macros
loci: remove extra_length

The "extra length" was the total length of members at non-fixed offsets. We're
going to include this in the base length instead.
ir: include external alignment when determining base length
loci: replace calculate_offsets_and_lengths with code using the IR

This function also searched for lists and special offsets. I moved this logic
to analyze_input.
loci: hack _PACKET_IN_DATA_OFFSET into working

This whole family of macros is deficient. They assume the match is the last
member, but of_packet_in has a 2-byte pad member between the match and the
data.