refactor(library/data): remove folders with a single file
refactor(library/data/nat): rename theorems
feat(library/data/nat): make nat an instance of comm_semiring
refactor(library): change mul.left_id to mul_one, and similarly for mul.right_id, add.left_id, add.right_id
refactor(library/data/int,library/algebra): make int an instnance of ordered ring, rename theorems
fix(library/data/nat/div,tests/lean/run/ppbeta): make decidable for dvd transparent, name change in ppbeta
fix(library/data/nat/order): delete unused material at end of file
fix(shell/lean): crash when trying to read file without extension
fix(frontends/lean/pp): option 'pp.private_names' should also affect private declarations defined in the current file