Remove usage of (require 'cl)

[gracjan]

Supposedly cl-lib is the way to go, so change those:

$ grep -Hn "require 'cl)" *.el
haskell-cabal.el:37:;;   (with-no-warnings (require 'cl))
haskell-doc.el:303:(eval-when-compile (require 'cl))

[fice-t]

From what I can tell, the only cl function used in haskell-doc is lexical-let, which when using lexical binding is useless.

[gracjan]

@fice-t: Can you fix those two places?