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.
what is this for?  what does f:f:... mean?  is it f0 or 0f or ff?