Offer module uses removed functionality from kcas package

[henrymercer]

Line 42 of lib/offer.ml references Kcas.get_id (), however this function has been removed in commit https://github.com/kayceesrk/kcas/commit/d13586b62bb6d5ff91ebddf180b564a1edf395bc to the kcas package.

https://github.com/ocamllabs/reagents/blob/69f5babbf731fd193d221fc437275d9d32b8a1d7/lib/offer.ml#L42