chore(library/Makefile): remove unused Makefile
feat(emacs/lean-type): add lean-show-type-add-to-kill-ring option

If a customization option 'lean-show-type-add-to-kill-ring' is non-nil
and a user runs 'lean-show-type' by explicitly typing 'C-c C-t',
lean-mode saves the displayed type information message into kill-ring
, which can be pasted by yank (C-y) command.

Close #245
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