Add more syntactic sugar for polynomial ring constructors

[SoongNoonien]

While reading a question by @lkastner on Slack I saw this polynomial ring construction:

julia> K = algebraic_closure(QQ)
Algebraic closure of rational field

julia> Kx, xx = K[:x]
(Univariate polynomial ring in x over QQBar, x)

julia> Kxy, yy = Kx[:y]
(Univariate polynomial ring in y over Kx, y)

And I thought that this should be a little more convenient like this:

julia> R, (x,y) = QQ[:x][:y]
(Univariate polynomial ring in y over univariate polynomial ring, AbstractAlgebra.Generic.Poly{AbstractAlgebra.Generic.Poly{Rational{BigInt}}}[x, y])

QQ[:x][:y] existed before this PR but it returned only the polynomial ring in y and the variable y itself. I've extended this idea to support all combinations of nested univariate and multivariate polynomial rings. While at it, I also noticed that we allow R[:x,:y] for non-commutative rings R even though there is no implementation of multivariate polynomial rings over non-commutative rings. (One can argue what this should even mean, I'd rather expect R[:x,:y] to be the free algebra on $x$ and $y$ over R.) Simply removing this method would cause QQ[:x] to create a multivariate polynomial ring in just one variable. So, I made the multivariate shorthand construction more explicit, such that at least two variables are needed for multivariate polynomial rings. If desired I can separate this change into another PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.95%. Comparing base (2980528) to head (7bc479c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2191   +/-   ##
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  Coverage   87.95%   87.95%           
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  Files         127      127           
  Lines       31791    31805   +14     
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+ Hits        27961    27975   +14     
  Misses       3830     3830           

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