Remove redundant _lmul_ and _rmul_ methods

[jdemeyer]

1. Assume commutativity by default and implement _rmul_ as falling back to _lmul_ (like we already do for AdditiveGroupElement).

2. Implement the basic _lmul_ and _rmul_ just once in ModuleElement: remove the redundant implementations from RingElement and AdditiveGroupElement.

Component: coercion

Author: Jeroen Demeyer

Branch/Commit: 69d18c7

Reviewer: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/21140

[jdemeyer]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
-Implement the basic `_lmul_` and `_rmul_` just once in `ModuleElement`. Remove the redundant implementations from `RingElement` and `AdditiveGroupElement`.
+1. Assume commutativity by default and implement `_rmul_` as falling back to `_lmul_` (like we already do for `AdditiveGroupElement`).
+
+2. Implement the basic `_lmul_` and `_rmul_` just once in `ModuleElement`: remove the redundant implementations from `RingElement` and `AdditiveGroupElement`.

[jdemeyer]

Branch: u/jdemeyer/remove_redundant__lmul__and__rmul__methods

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

69d18c7

Remove redundant `_lmul_` and `_rmul_` methods

[sagetrac-git]

Commit: 69d18c7

[videlec]

Reviewer: Vincent Delecroix

[vbraun]

Changed branch from u/jdemeyer/remove_redundant__lmul__and__rmul__methods to 69d18c7