Revert "use internal polymorphic compare"

This reverts commit 2ddc66ef7f0967e30971a1d3b884d2f59444da9f.

Re: https://github.com/mirleft/ocaml-tls/pull/25 : although compare does work that way, I'll go with this man's opinion any day: http://stackoverflow.com/a/20348513 .

So I concur: it is underspecified.