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.
lexer.check('mask') consumes the expected token ('mask') if success, so there was one spared lexer.next().