Turn FinGenAbGroupElem into a non-mutable struct

[fingolfin]

This reduce allocations and increases speed of code using these elements.

Using the same example as in oscar-system/Oscar.jl#4119, we get the following.

Before this PR here:

julia> @btime is_homogeneous(f);
  1.533 μs (62 allocations: 2.28 KiB)

julia> @btime is_homogeneous(F);
  3.932 μs (152 allocations: 5.53 KiB)

With just this PR:

julia> @btime is_homogeneous(f);
  1.404 μs (48 allocations: 2.00 KiB)

julia> @btime is_homogeneous(F);
  3.688 μs (116 allocations: 4.56 KiB)

With just oscar-system/Oscar.jl#4119:

julia> @btime is_homogeneous(f);
  334.071 ns (10 allocations: 416 bytes)

julia> @btime is_homogeneous(F);
  556.150 ns (12 allocations: 656 bytes)

With this PR and oscar-system/Oscar.jl#4119:

julia> @btime is_homogeneous(f);
  330.772 ns (8 allocations: 352 bytes)

julia> @btime is_homogeneous(F);
  564.865 ns (10 allocations: 592 bytes)

So allocations are further reduced but performance stays the same. But that's OK because in that example only relatively few FinGenAbGroupElem are used after oscar-system/Oscar.jl#4119 ; but many other places still use them extensively, so it still seems quite useful to me to have this.

[lgoettgens]

What is this change about? I thought QadicField is deprecated

[fingolfin]

Huh, that should not be here. Looks like by accident some old file content snuck in. Will fix it ASAP

[thofma]

I don't see the harm. I guess many more element objects could/should be immutable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.85%. Comparing base (d9b7787) to head (3bcb770).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1621      +/-   ##
==========================================
+ Coverage   75.78%   75.85%   +0.07%     
==========================================
  Files         361      361              
  Lines      113695   113692       -3     
==========================================
+ Hits        86164    86242      +78     
+ Misses      27531    27450      -81     

Files with missing lines	Coverage Δ
src/GrpAb/Elem.jl	96.64% <ø> (-0.11%)	⬇️
src/HeckeTypes.jl	84.40% <100.00%> (+0.03%)	⬆️

... and 34 files with indirect coverage changes