Add missing parentheses in stacked printing of univpolys

[lgoettgens]

Consider the following example:

julia> R = universal_polynomial_ring(QQ)
Universal Polynomial Ring over Rationals

julia> x = gen(R, "x")
x

julia> S, y = polynomial_ring(R, "y")
(Univariate polynomial ring in y over universal Polynomial Ring over Rationals, y)

julia> (x+1)*y
x + 1*y

The missing parentheses in the last output are clearly misleading, and create an ambiguity between (x+1)*y and x+1*y.

This PR addresses this issue by replacing the printing of univ polys by using the expressify output of the underlying MPolyRing. This then fixes the recursive call of expressify on the coefficients of the polyring S.

Let's hope that this behavior is not doctested downstream and can be released quickly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.88%. Comparing base (2da9299) to head (8290c05).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1667      +/-   ##
==========================================
- Coverage   86.88%   86.88%   -0.01%     
==========================================
  Files         116      116              
  Lines       29652    29650       -2     
==========================================
- Hits        25764    25762       -2     
  Misses       3888     3888              

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