* guix/import/cabal.scm (is-test): Allow spaces between keyword and
parentheses.
(is-id): Add argument 'port'. Allow spaces between keyword and column.
(lex-word): Adjust call to 'is-id'.
* guix/import/cabal.scm (is-true, is-false, lex-true, lex-false): New procedures.
(lex-word): Use them.
(make-cabal-parser): Add TRUE and FALSE tokens.
(eval): Add entries for 'true and 'false symbols.
* 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'.