Summary:
We need equality over Types, etc; and this equality has to be modulo alpha
renaming. Previously, we baked CmEnv into the generic "empty, singleton, many"
structure. This isn't great, really GenMap should be more generic than that.

The insight: we've defined the key wrong: the key should be *equipped*
with the alpha-renaming information (CmEnv) and a TrieMap queried with this.
This is what the DeBruijn constructor does.

Now, when we define TrieMap instances, we don't have to synthesize an emptyCME
to pass to the helper functions: we have all the information we need. To make a
recursive call, we construct a new DeBruijn with the updated CME and then
call lookupTM on that. We can even define a plain old Eq instance on DeBruijn
respecting alpha-renaming.  There are number of other good knock-on effects.

This patch does add a bit of boxing and unboxing, but nothing the optimizer
shouldn't be able to eliminate.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D608

GHC Trac Issues: #9960