Implement operators 'less' and 'greater'

* doc/Makefile: Add examples of 'less' and 'greater'.
* doc/dst-host-192.68.1.1-and-greater-100.md: Example of 'greater'.
* doc/src-host-192.68.1.1-and-less-100.md: Example of 'less'.
* src/pf/codegen.lua: Add operator 'len'.
* src/pf/expand.lua: Implement expansion for 'less' and 'greater'.

I think I prefer the function down here.  Can you just move up the "local parse_arithmetic" declaration?  Cheers.