Revert exponent rationalization efforts

[SoongNoonien]

I was a bit quick with those. While converting the 2B2 table locally to rational exponents I noticed many other problems that prevented importing. Finally I could resolve all of these just to notice that the whole concept is flawed. Take a look at nesum where it is mathematically necessary to have a well defined multiplication of modulusring elements into the argumentring. We could work around these by using the mapping between q and q0 but I think this makes things way to complicated.
My new idea is now to fully integrate the congruence field of the tables into the simplification of the cyclotomics. All tables with nonrational expressions in their character values have a congruence which ensures rationality in the exponents. This way we can use something like change_base_ring to ensure rationality while applying the normal form algorithms.

@fingolfin I'm not sure if this is how you like reverts to be done but I didn't want to force push to master.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.37%. Comparing base (b4e11a4) to head (8707ee8).

Files	Patch %	Lines
src/Arith.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   70.45%   70.37%   -0.08%     
==========================================
  Files          10       10              
  Lines         907      908       +1     
==========================================
  Hits          639      639              
- Misses        268      269       +1     

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[fingolfin]

OK, let's talk more about it next week (I'll be back in KL only on Thursday and that day is already "full")