Allow fraction coercion to residue rings

[SoongNoonien]

I think this should be there alongside the other coercions we support already.

[fingolfin]

I note that this also works in Oscar:

julia> F = GF(5);

julia> F(2//3)
4

And of course GAP also supports all that, so this change is fine from my side... But let's see if @thofma or @fieker agree / disagree.

[thofma]

Fine with me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.07%. Comparing base (290941e) to head (eeab579).
⚠️ Report is 41 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2245      +/-   ##
==========================================
+ Coverage   88.00%   88.07%   +0.06%     
==========================================
  Files         127      127              
  Lines       31803    31773      -30     
==========================================
- Hits        27989    27984       -5     
+ Misses       3814     3789      -25     

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[fieker]

as long as this does not automatically involce to allow (2//3) mod 5 I am happy