Add codegen optional name arg

* src/pf/codegen.lua: Second optional argument indicating the name for
  the loaded filter.
Simplify packet access

* src/pf/codegen.lua: Simplify accessing packet data.
Packet access in host endianness, optimize out swaps

* src/pf/codegen.lua: Access shorts and longs using the FFI API.  It's a
  very slight slowdown but the right answer here is to fix the JIT.  Add
  support for the ntohs and ntohl unary ops, though they are usually
  optimized out.

* src/pf/expand.lua: Change the meaning of [] to mean host-order access,
  and add ntohl / ntohs.  Add optimizations to remove these if
  possible.