refactor(library/tactic/goal): remove unnecessary parameter
feat(library/tactic/apply_tactic): modify heuristic for adding arguments to apply tactic.
feat(library/tactic): add generalize tactic, closes #34

Remark: the intros tactic has been added in a different commit: 7d0100a3400b70c19422c
feat(library/tactic): add generalize tactic, closes #34