Added make rule for omitting code generation

[EliasC]

It turns out that the most expensive part of building encorec is the
code generation phases. The GHC option -fno-code stops after
typechecking, and on my machine the difference in runtime is 5 seconds
vs. ~1.45 minutes. This commit adds the rule typecheck to the root
makefile, which meant to be used when no executable needs to be
generated (i.e. for only checking code that you wrote).

Emacs users using haskell-mode might be interested in the following hack
to make the compilation command default to only typechecking, and also
try to start a new haskell session if no one is present:

(defun ec/haskell-typecheck ()
  (interactive)
  (progn
    (cond ((null haskell-session) (haskell-session-change)))
    (haskell-process-do-cabal "build --ghc-options=-fno-code")))

(add-hook 'haskell-mode-hook
   (function
    (lambda ()
      (local-set-key (kbd "C-c C-c") 'ec/haskell-typecheck))))

[EliasC]

No idea why CI fails, it builds fine on my machine.

[kikofernandez]

it is an old error, if you re-push it should be fine

[albertnetymk]

Looks all right. Merging if no objections.