gh-38729: provide monomial_coefficients for polynomials and allow single argument for MPolynomialRing_base.monomial
The goal of this pull request is to allow generic programming with
instances of `CombinatorialFreeModule` and polynomial rings.
In particular, we provide an alias `monomial_coefficients` for the
existing method `dict` of polynomial rings, and we allow to specify the
exponents in `monomial` of multivariate polynomial rings as a single
tuple.
URL: https://github.com/sagemath/sage/pull/38729
Reported by: Martin Rubey
Reviewer(s): Frédéric Chapoton, Martin Rubey, Travis Scrimshaw
