Fixes <https://debbugs.gnu.org/32387>.
* guix/import/cabal.scm (eval-cabal): Avoid mis-match when the custom-setup
section cannot be created.
* guix/import/hackage.scm (cabal-custom-setup-dependencies->names): Do not
crash when cabal-package-custom-setup returns #F.
Update ghc-standard-libraries to match the output of `ghc-pkg list` when
using GHC 7.10.2.
* guix/import/hackage.scm (ghc-standard-libraries): Sort and update list
of core GHC libraries.
* guix/import/cabal.scm: New file.
* guix/import/hackage.scm: Update to use the new Cabal parsing module.
* tests/hackage.scm: Update tests.
* guix/scripts/import/hackage.scm: Add new '--cabal-environment' and '--stdin'
options.
* doc/guix.texi: ... and document them.
* Makefile.am (MODULES): Add 'guix/import/cabal.scm',
'guix/import/hackage.scm' and 'guix/scripts/import/hackage.scm'.
(SCM_TESTS): Add 'tests/hackage.scm'.