refactor(ring_theory/polynomial/homogeneous): redefine `mv_polynomial.homogeneous_component` (#5294)
refactor(ring_theory/polynomial/homogeneous): redefine `mv_polynomial.homogeneous_component` (#5294)

* redefine `homogeneous_component` using `finsupp.restrict_dom`,
  “upgrade” it to a `linear_map`;

* add `coeff_homogeneous_component` and use it to golf some proofs.





leanprover-community/mathlib,leanprover-community/mathlib