base: Enable listToMaybe to fuse via foldr/build

Test Plan: Consider whether this is a good idea.

Reviewers: austin, hvr, bgamari, nomeata

Reviewed By: bgamari, nomeata

Subscribers: nomeata, rwbarton, thomie

GHC Trac Issues: #14387

Differential Revision: https://phabricator.haskell.org/D4126