feat(frontends/lean/info_manager): add proof_state info
feat(library/reducible): expose 'memoize' flag
fix(kernel/type_checker): propagate 'memoize' flag to default_converter
feat(frontends/lean): add 'info' tactic for producing PROOF_STATE info for emacs mode
feat(frontends/lean): automatically add 'info' tactic in begin-end blocks

Actually, the tactic is only added when Lean is in collect-info mode.
feat(frontends/lean): automatically add 'info' tactic in begin-end blocks