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.
Yes, the value of addr[i] will be a number in the range of 0-65535, but addr[i] is still a Lua number and it's possible to store values higher than 16-bit. I use 0x10000 as a flag to indicate a row of zeros (for instance ffff:eeee::8888:1, is coded as ffff:eeee:0x10000:8888:1).