mathlib

51f7319b - chore(algebra/module): rename type vars, minor cleanup, add module docstring (#2392)

chore(algebra/module): rename type vars, minor cleanup, add module docstring (#2392) * Use `R`, `S` for rings, `k` for a field, `M`, `M₂` etc for modules; * Add a `semiring` version of `ring_hom.to_module`; * Stop using `{rα : ring α}` trick as Lean 3.7 tries unification before class search; * Add a short module docstring