HACK: add faster arithmetic for UnivPoly and ZZRingElem

[fingolfin]

This is not meant to be merged, just serve as a reminder: arithmetic for UnivPoly could be quite a bit more efficient. The methods added here help concrete computations in the method (R::GenericCycloRing)(f::Dict{UPolyFrac, UPoly}; simplify::Bool=true).

But they are not going far enough and are incomplete. Someone should patch AA (and perhaps Nemo) to add...

• add!, sub!, mul! methods in AA looking like this:

  function add!(a::UnivPoly{T}, b::UnivPoly{T}, c::Any) where {T <: RingElement}
     a.p = add!(a.p, b.p, c)
     return a
  end

  (One may argue whether c should be restricted to e.g. RingElement but I think for this kind of fallback it has no advantage?)
• we should also add addmul! (there also in the variant which takes three UnivPoly)
• then of course improved +, -, * etc. methods possibly based on the new add!, sub!, mul! methods

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.44%. Comparing base (32a11a5) to head (f739fa1).

Files with missing lines	Patch %	Lines
src/GenericCyclotomics.jl	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
- Coverage   95.77%   95.44%   -0.33%     
==========================================
  Files          13       13              
  Lines         781      791      +10     
==========================================
+ Hits          748      755       +7     
- Misses         33       36       +3     

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