Implement lowering of 'host' and 'net' for IPv6

* tools/pflang-compile: Escaped tabs ('\t') are printed as raw text within a long string. Use non-escaped tabs instead.
* src/pf/parse.lua: Improve parsing of IPv6 addresses. Support not full IPv6 addresses (::1, 0::0, etc). Improve mask handling in IPv6 addresses.
* src/pf/expand.lua: Add lowering of 'host' and 'net' for IPv6 addresses. I'm not sure what's the equivalent of BPF 'jset' operation, see FIXME.
* src/pf/utils.lua: Add method for converting a 16-bit 8-groups IPv6 address into a 32-bit 4-groups address. Comparison of parts of an IPv6 address is done by fetching groups of 32bit.
Not really directly with determining whether the address is IPv6 or not. In fact the '/len' suffix is not necessary for IPv6 because is the only way possible to code a mask, but I think is good to leave like that for consistency.