doc/faq/integers: link to Dan Luu's blog post on checked arithmetic

Also change the text of the final paragraph of this FAQ entry.
added isless() for IPAddrs - enables sorting and comparisons

made isless() a one-liner

make sure types are the same, and add some tests

more tests
Merge pull request #9399 from sbromberger/ipaddr-isless

added isless() for IPAddrs - enables sorting and comparisons