Eric Mertens pointed out that using the default `foldMap`
implementation for `Maybe` led to an efficiency problem by
implementing `foldMap f (Just x)` as `f x <> mempty` rather than
as `f x`. This should solve the problem.

Reviewers: hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: glguy, thomie

Differential Revision: https://phabricator.haskell.org/D2988