Use a ReaderT in TcDeriv to avoid some tedious plumbing
Addresses point (2) of https://phabricator.haskell.org/D3337#107865.
Before, several functions in TcDeriv and TcDerivInfer which compute
an EarlyDerivSpec were manually threading through about 10 different
arguments, which contribute to quite a lot of clutter whenever they need
to be updated. To minimize this plumbing, and to make it clearer which
of these 10 values are being used where, I refactored the code in
TcDeriv and TcDerivInfer to use a new DerivM type:
type DerivM = ReaderT DerivEnv TcRn
where DerivEnv contains the 10 aforementioned values. In addition to
cleaning up the code, this should make some subsequent changes planned
for later less noisy.
Test Plan: ./validate
Reviewers: austin, bgamari
Subscribers: rwbarton, thomie
Differential Revision: https://phabricator.haskell.org/D3846