Def span.repr

chore(*): remove after the fact `attribute [irreducible]` at several places (#18168) …

feat(linear_algebra/dimension): generalize inequalities and invariance of dimension to arbitrary rings (#8343) …

feat(linear_algebra/finsupp): `span.repr` gives an arbitrarily representation of `x : span R w` as a linear combination over `w` (#8339) …