Currently, `-XDeriveFoldable` and `-XDeriveTraversable` generate
unnecessary `mempty` and `pure` expressions when it traverses of an
argument of a constructor whose type does not mention the last type
parameter. Not only is this inefficient, but it prevents `Traversable`
from being derivable for datatypes with unlifted arguments (see
Trac #11174).

The solution to this problem is to adopt a slight change to the
algorithms for `-XDeriveFoldable` and `-XDeriveTraversable`, which is
described in [this wiki
page](https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFu
nctor#Proposal:alternativestrategyforderivingFoldableandTraversable).
The wiki page also describes why we don't apply the same changes to the
algorithm for `-XDeriveFunctor`.

This is techincally a breaking change for users of `-XDeriveFoldable`
and `-XDeriveTraversable`, since if someone was using a law-breaking
`Monoid` instance with a derived `Foldable` instance (i.e., one where `x
<> mempty` does not equal `x`) or a law-breaking `Applicative` instance
with a derived `Traversable` instance, then the new generated code could
result in different behavior. I suspect the number of scenarios like
this is very small, and the onus really should be on those users to fix
up their `Monoid`/`Applicative` instances.

Fixes #11174.

Test Plan: ./validate

Reviewers: hvr, simonpj, austin, bgamari

Reviewed By: simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1908

GHC Trac Issues: #11174