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
refactor(library/unifier): remove dead code
fix(library/unifier): postpone class-instance constraints whose type could not be inferred
feat(library/hott) began porting floris' category implementation
continued trying to replace the proof irrelevance
fix(library/hott) finished associativity proof
feat(library/hott) finished porting of natural_transformation.lean
feat(library/hott) ported a part of algebra/category/constructions.lean, slice category still to do
feat(library/hott) added iso_of_path lemma for precategories
chore(library/hott) move precategory definition to its own folder
chore(library/algebra) change category to be a structure
fix(library/hott) adjust expliciteness of arguments
chore(library/hott) chang precategory to structure, fix morphism.lean