Specifying parent for minpoly and charpoly

[fingolfin]

The parent ring for minimal and characteristic polynomial is in general specified by passing it as the first argument of these functions.

There is one exception where one can also pass it as a kwarg:

function characteristic_polynomial(M::Matroid;
           parent::ZZPolyRing = polynomial_ring(ZZ, :q; cached = false)[1])
  return (-1)^rank(M) * tutte_polynomial(M)(1 - gen(parent), 0)
end

We had a long discussion about this back in the day...

I still think there is merit to allowing this variant, but it is really confusing that we only allow it in this one instance. It'd be better if we allowed this in general.

To do so, we'd have to modify all single argument versions of those two to accept an optional additional kwarg.

If my proposal in #4518 is implemented, there'd be only a few (or even only one) 1-arg method each, so adding the optional parent kwarg in that case would be trivial.