refactor(library/init): move num->nat coercion to init
test(tests/lean/slow): add "manual" 'path induction' tactic
feat(frontends/lean/pp): add hard-coded pretty printer for nat numerals
feat(library/definitional/brec_on): simplify universe level constraints for non-reflexive recursive datatypes
fix(tests/lean): to reflect recent changes
chore(frontends/lean/pp): fix style
fix(library/tactic/inversion_tactic): bug in mutually recursive case
fix(library/tactic/inversion_tactic): add missing case