Fix "overlapping domain" Dialyzer warning for '#is_record-'/{1,2}

These contracts will usually not be seen by the user, and are ignored
by Dialyzer because they have overlapping domains, so there is not
much point in using them.  Replace with simpler contracts for now.
Merge pull request #7 from legoscia/is_record_overlapping_contracts

Fix "overlapping domain" Dialyzer warning for '#is_record-'/{1,2}