Prefer symbols for VarNames over strings

[fingolfin]

Both are valid and allowed and will remain so. But using symbols overall
avoids a bunch of allocations, so it makes sense to adjust our internal
code as well as user-facing documentation to use symbols to set a good
precedent.

To demonstrate the effect, here a benchmark with strings:

julia> @btime polynomial_ring(QQ, ["x", "y"]);
  756.274 ns (27 allocations: 1.22 KiB)

Here the same with symbols:

julia> @btime polynomial_ring(QQ, [:x, :y]);
  535.963 ns (20 allocations: 896 bytes)

Of course code where this noticeable would be code that calls polynomial_ring
far too many times, and should be changed to not do that.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.02%. Comparing base (fd3af98) to head (5d22a95).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Rings.jl	0.00%	1 Missing ⚠️
src/algorithms/MPolyFactor.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1804   +/-   ##
=======================================
  Coverage   88.02%   88.02%           
=======================================
  Files         119      119           
  Lines       30057    30057           
=======================================
+ Hits        26457    26459    +2     
+ Misses       3600     3598    -2     

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[lgoettgens]

Looks good, but there are doctest failures