Unify univariate and multivariate power series rings, as has been done for polynomial rings #9981
Description
Multivariate power series rings are implemented in #1956 as a subclass of (univariate) power series rings. The two should be merged, following the implementation of polynomial rings. As a first step, some behavior of PowerSeriesRing must be deprecated; a ticket for this is at #9980.
CC: @nilesjohnson
Component: commutative algebra
Keywords: power series
Issue created by migration from https://trac.sagemath.org/ticket/9981