refactor(library/tactic): cleanup 'revert' and 'clear' tactics
refactor(library/tactic): move 'get_unused_name' to goal
feat(library/rename): add notation for `rename`
refactor(library/tactic/intros_tactic): change approach for generating fresh names for nameless 'intros'
feat(library/tactic): add first step of 'inversion' tactic
refactor(library/tactic/inversion_tactic): add 'cases_on' step to inversion_tactic
refactor(library/tactic/inversion_tactic): add 'cases_on' step to inversion_tactic