INLINE unfoldr

Summary:
to allow GHC to maybe remove the Maybe. See the code comment for more
commentary. This fixes #9369.

Test Plan: see what happens on ghcspeed (once it is merged)

Reviewers: austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D198

GHC Trac Issues: #9369