Add some specialized methods for universal polynomials

[SoongNoonien]

As @fingolfin mentioned in Nemocas/AbstractAlgebra.jl#1753 (comment), it would be nice to have some methods for certain universal polynomial rings. This is a first implementation of denominator. I think there should be some more methods that are useful. But I haven't found them yet as I currently only need denominator.

Resolves oscar-system/GenericCharacterTables.jl#203

[fingolfin]

Some other methods you should perhaps add are here: oscar-system/GenericCharacterTables.jl#203

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.29%. Comparing base (82193e6) to head (bddd8cc).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2013      +/-   ##
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- Coverage   88.31%   88.29%   -0.02%     
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  Files          98       99       +1     
  Lines       36126    36196      +70     
==========================================
+ Hits        31903    31959      +56     
- Misses       4223     4237      +14     

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[fingolfin]

This looks sensible to me. In case @lgoettgens has time and interested to have a quick look, I'll wait a bit more with merging this.

[lgoettgens]

Wouldn't all of these here make sense for, let's say, all "scalar" types that have ad-hoc operations?

In particular, I am thinking about Integer and T (the coeff type), but I guess those would belong in AA instead.

[fingolfin]

Such methods already exist for Integer and T in AA. But not for ZZRingElem for obvious reasons. Which is why we add them here.

[fingolfin]

Example demonstrating the effect:

julia> S = universal_polynomial_ring(QQ)
Universal Polynomial Ring over Rational field

julia> x, y = gens(S, [:x,:y])
(x, y)

julia> f = x^3 + y
x^3 + y

julia> n = ZZ(1)
1

Then in master we get:

julia> @b $f  + 1
209.213 ns (6 allocs: 208 bytes)

julia> @b $f  + $n
327.180 ns (10 allocs: 408 bytes)

julia> @b $f.p  + $n
205.153 ns (5 allocs: 176 bytes)

With this branch:

julia> @b $f  + 1
207.000 ns (6 allocs: 208 bytes)

julia> @b $f  + $n
214.286 ns (6 allocs: 208 bytes)

julia> @b $f.p  + $n
203.224 ns (5 allocs: 176 bytes)