Enable Aqua.jl's ambiguity check

[lgoettgens]

Parts of this have already been merged as #937.

• 470bde7 removes (::Type{<:AbstractString})(::GapObj). This led to ambiguities with Test.GenericString, LazyString, Core.Compiler.LazyString. The only real removals here are for SubString and SubstitutionString as the other subtypes of AbstractString either have their specialized function or didn't work anyway due to ambiguities.
  It furthermore removes the unclear gap_to_julia(AbstractString, ...) (as it didn't specify what exactly to return). The implementation chose to return a simple String. I made that a bit more clear. Even the documentation says that one should put String as the first argument (cf.

  GAP.jl/src/gap_to_julia.jl

  Line 78 in 01b5c13

  | `IsStringRep` | `String` | `Symbol`, `Vector{T}` |

  ).
• 1291ab0 fixes all calls of it and the corresponding tests.

Due to the AbstractString stuff above, I would technically consider this breaking. (E.g. SubString(GAP.evalstr("\"foo\"")) used to work, but now needs to be SubString(String(GAP.evalstr("\"foo\""))).)

Once this is merged, GAP.jl fulfills all of Aqua.jl's tests! 🎉

[lgoettgens]

There seems to be more behind this as initially expectes. I continue later.

[fingolfin]

I am really uncomfortable with using applicable here, IMHO this results in difficult to understand and handle code.

I think a more appropriate fix is to just do

Suggested change

	gap_to_julia(::Type{T}, obj::GapObj) where {T<:AbstractFloat} = applicable(T, obj) ? T(obj) : T(Float64(obj))
	gap_to_julia(::Type{Float64}, obj::GapObj) = Float64(obj)

and anyone who wants to convert to stuff to Float32 or BigFloat has to deal with it by themselves (e.g. for a GAP big int it's a bad idea to first convert to Float64 in that case).

[lgoettgens]

After looking again through the Float stuff, it seems that it caused no ambiguities, and it is fundamentally different to the removed String stuff, as it does not use a magic convert. So this doesn't need to be changed at all.

[fingolfin]

OK, great! (Sorry, didn't see this comment before)

[codecov]

Codecov Report

Merging #936 (6e57e62) into master (b216185) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #936      +/-   ##
==========================================
- Coverage   75.64%   75.62%   -0.02%     
==========================================
  Files          51       51              
  Lines        4171     4169       -2     
==========================================
- Hits         3155     3153       -2     
  Misses       1016     1016              

Files Changed	Coverage
src/constructors.jl	ø
src/gap_to_julia.jl	100.00%