Added script for generating BPF asm and Lua code generated by libpcap and
pflua.

- Generate BPF.

        $ pflang-compile --bpf "tcp port 80"

- Generate libpcap Lua code.

        $ pflang-compile --lua "tcp port 80"

- Generate pflua Lua code.

        $ pflang-compile --pflua "tcp port 80"

The flags are not exclusive, they can be combined to get several outputs
at the same time.

Some code reorganization within 'tools/' folder.
Expand 'host <host>'

   - Add support for parsing: "[ip|arp|rarp] host <host>".
   - Add lowering for "[ip|arp|rarp]? host <host>".
   - For the moment only support <host> as IPv4 addresses.
   - Lowering of "host <host>" is not correct.
Implement 'src host' and 'dst host'