Currently, DefaultCovertMaps have an attribute _is_coercion that is unused. I propose moving it up to Map and setting it when coercion maps are created.

Component: coercion

Keywords: sd86.5

Author: David Roe, Julian Rüth

Branch: 78807fa

Reviewer: Julian Rüth, David Roe

Issue created by migration from https://trac.sagemath.org/ticket/23211