Why is `IndexStyle` specialized for `Base.OneTo`?

https://github.com/JuliaArrays/FillArrays.jl/blob/de36a50a171a4f871d689762ba7886198dd96923/src/FillArrays.jl#L59

This means
```julia
julia> using StaticArrays

julia> f = Fill(1, (SOneTo(2),))
2-element Fill{Int64, 1, Tuple{SOneTo{2}}} with indices SOneTo(2), with entries equal to 1

julia> IndexStyle(f)
IndexCartesian()
```
This may use linear indexing as well, though.