feat(emacs/Cask): add Cask file

Close #92
feat(emacs/lean-require): check lean-mode dependencies in initialization

Close #91
doc(emacs/README.md): update required/optional packages
fix(frontends/lean/elaborator): whnf may produce assertion violation if term contains free variables
fix(frontends/lean/class): take whnf into account in add_instance
refactor(library): replace decidable_eq with abbreviation