Change signature of / and \ methods

[fingolfin]

... to match those of their counterpart functions divexact_right and
divexact_left: it seems more natural to expect to be able to divide
a ring element by floats or rationals than the other way around.

[codecov]

Codecov Report

Merging #1480 (df4da39) into master (b5090f4) will increase coverage by 2.09%.
Report is 3 commits behind head on master.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1480      +/-   ##
==========================================
+ Coverage   84.76%   86.86%   +2.09%     
==========================================
  Files         110      110              
  Lines       29370    28672     -698     
==========================================
+ Hits        24895    24905      +10     
+ Misses       4475     3767     -708     

Files	Coverage Δ
src/NCRings.jl	89.24% <0.00%> (ø)
src/Rings.jl	52.54% <0.00%> (-0.91%)	⬇️

... and 3 files with indirect coverage changes

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[fingolfin]

Grr, while this fixes a JET warning, it introduces ambiguities:

ERROR: LoadError: LoadError: LoadError: MethodError: /(::Nemo.fpFieldElem, ::Int64) is ambiguous. Candidates:
  /(x::AbstractAlgebra.NCRingElem, y::Union{AbstractFloat, Integer, Rational}) in AbstractAlgebra at /home/runner/work/AbstractAlgebra.jl/AbstractAlgebra.jl/src/NCRings.jl:123
  /(x::AbstractAlgebra.RingElem, y::AbstractAlgebra.RingElement) in AbstractAlgebra at /home/runner/work/AbstractAlgebra.jl/AbstractAlgebra.jl/src/Rings.jl:34
Possible fix, define
  /(::AbstractAlgebra.RingElem, ::Union{AbstractFloat, Integer, Rational})