Implement missing lowering operators

Currently only a small subset of lowering operators are implemented.  This set should be made complete.  Happily it's not too difficult -- you just compile an expression with libpcap, cross-compile to Lua using the pflua bpf compiler, look at the generated code, then make the pflang compiler do something similar.
Yes, that's correct. Parsing of IPv6 addresses needs more work, there's a FIXME in the code parse.lua:57. Maybe for the moment is better to match a MAC address which has a more fixed format and if not anything "^::%x" | "^%x%x?%x?%x?:" is an IPv6 address.

I was concerned whether a MAC address could be mixed up with an IPv6 address as it's possible to leave fields empty in an IPv6 address if the value of all octets are 0, but I think is not possible as in that case there will be two semicolons one after another. For instance:

MAC: ff:ff:ff:33:33:33
IPv6: ff:ff:ff:33:33:33:: (same as 00ff:00ff:00ff:0033:0033:0033:0000:0000)