FinField: use concrete types for sub-/overfield morphisms to improve inferrability

[benlorenz]

Both functions modifying these dicts already restrict the types in the same way:

function AddOverfield!(F::T, f::FinFieldMorphism{T, T}) where T <: FinField
function AddSubfield!(F::T, f::FinFieldMorphism{T, T}) where T <: FinField

Without this PR:

julia> using Oscar
...
julia> F = finite_field(3)[1]
Prime field of characteristic 3

julia> a = F(0)
0

julia> @time @eval a*a
 48.451252 seconds (221.69 M allocations: 10.308 GiB, 6.01% gc time, 99.98% compilation time: 100% of which was recompilation)
0

(Any further calls will be fast since it is already compiled)

With the changes in this PR:

julia> using Oscar
...
julia> F = finite_field(3)[1]
Prime field of characteristic 3

julia> a = F(0)
0

julia> @time @eval a*a
  0.000158 seconds (56 allocations: 2.203 KiB)
0

Without the changes here the return type of Nemo.domain(::FinFieldMorphism) which is used inside Nemo.intersections(::FinField,::FinField) cannot be inferred. Then the compiler seems to look at all possible embed(Any,Any) functions which takes very long, especially the branch for Hecke.embed(S::ZZLat, G::ZZGenus).

For Nemo alone the difference is less significant, but it does improve allocations during compilation quite a bit.
0.47.5:

julia> @time @eval a*a
  0.806358 seconds (2.61 M allocations: 128.606 MiB, 2.89% gc time, 99.97% compilation time)
0

this PR:

julia> @time @eval a*a
  0.836485 seconds (1.29 M allocations: 63.900 MiB, 1.45% gc time, 99.97% compilation time)
0

cc: @flenzen @thofma @fingolfin

flamegraph for snoop_inference without this PR:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.93%. Comparing base (578b513) to head (a0b6f9a).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1961      +/-   ##
==========================================
- Coverage   87.96%   87.93%   -0.03%     
==========================================
  Files          99       99              
  Lines       36457    36457              
==========================================
- Hits        32069    32059      -10     
- Misses       4388     4398      +10     

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[fingolfin]

Fantastic!