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.
For "net <net> mask <mask>" the keyword is the same but the mask parameter is coded as part of the address parameter. For instance:

net 192.168.1.0 mask 255.255.255.0 => { net, { 'ipv4/mask', 192.168.1.0, 255.255.255.0 } }

So the expand_net function should handle this case too.