haskell: add three new helper functions to the lib module

 - sdistTarball transforms a normal Haskell build into one that produces a
   release tarball by running "sdist". For example:

     $ nix-shell -p "haskell.lib.sdistTarball haskellPackages.mtl" --command 'tar tfv $nativeBuildInputs/*tar.gz'

 - buildFromSdist transforms a normal Haskell build into one that compiles the
   package the an sdist release tarball created by Nix. For example:

     $ nix-shell -p "haskell.lib.buildFromSdist haskellPackages.mtl" --command "exit 0"

 - buildStrictly transforms a normal Haskell build into one that (a) compiles
   the source code with "-Wall -Werror" flags and (b) uses as input a locally
   generated sdist release tarball. For example:

     $ nix-shell -p "haskell.lib.buildStrictly haskellPackages.mtl" --command "exit 0"

   This function is particularly useful for continuous Hydra builds of Haskell
   packages.