We can sometimes produce much better code by deriving the
definition of `null` rather than using the default. For example,
given

    data SnocList a = Lin | Snoc (SnocList a) a

the default definition of `null` will walk the whole list, but of
course we can stop as soon as we see `Snoc`. Similarly, if a
constructor contains some other `Foldable` type, we want to use its
`null` rather than folding over the structure.

Partially fixes Trac #13280

Reviewers: austin, bgamari, RyanGlScott

Reviewed By: RyanGlScott

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3402