Add fraction_field_type and define fraction_field method for fields

[fingolfin]

Seems natural and I see no harm in it?

[codecov]

Codecov Report

Attention: Patch coverage is 81.48148% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.33%. Comparing base (8ea17bc) to head (11dd456).

Files with missing lines	Patch %	Lines
src/Fraction.jl	55.55%	4 Missing ⚠️
src/generic/RationalFunctionField.jl	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2092      +/-   ##
==========================================
- Coverage   88.34%   88.33%   -0.01%     
==========================================
  Files         126      126              
  Lines       31674    31679       +5     
==========================================
+ Hits        27982    27984       +2     
- Misses       3692     3695       +3     

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[lgoettgens]

Can we then please also have some programmatic way for deciding the type of the fraction field of some ring, i.e. fraction_field_type that usually returns some Generic.FracField{T} thingy, but T for T being a field subtype?

[fingolfin]

I agree with the idea of adding fraction_field_type, but while looking into it I found a more serious issue:

function fraction_field(a::RationalFunctionField{T, U}) where {T <: FieldElement, U <: Union{PolyRingElem, MPolyRingElem}}
   return a.fraction_field::Union{FracField{U}}
end

And this is also documented, so we can't simply change it.

But I think that kind of throws a wrench into the idea behind this PR sigh; at the very least it would require a breaking release which renames the above fraction_field getter to e.g. underlying_fraction_field (actually I would have expect this to be base_ring, but base_ring on a RationalFunctionField is the base ring / coefficientring of the underlying fraction field / polynomial ring... sigh

[lgoettgens]

Suggested change

	Return the type of the base ring of the given element, element type, parent or parent type $a$.
	Return the type of the fraction field of the given element, element type, parent or parent type $a$.