Ryan Scott authored Sep 28, 2019 and Marge Bot committed Oct 01, 2019

* `mk_eqn_stock`, `mk_eqn_anyclass`, and `mk_eqn_no_mechanism` all
  took a continuation of type
  `DerivSpecMechanism -> DerivM EarlyDerivSpec` to represent its
  primary control flow. However, in practice this continuation was
  always instantiated with the `mk_originative_eqn` function, so
  there's not much point in making this be a continuation in the
  first place.

  This patch removes these continuations in favor of invoking
  `mk_originative_eqn` directly, which is simpler.
* There were several parts of `TcDeriv` that took different code
  paths if compiling an `.hs-boot` file. But this is silly, because
  ever since 101a8c77 we simply error
  eagerly whenever attempting to derive any instances in an
  `.hs-boot` file.

  This patch removes all of the unnecessary `.hs-boot` code paths,
  leaving only one (which errors out).
* Remove various error continuation arguments from `mk_eqn_stock`
  and related functions.