improve cat design / performance (#49322)
This used to make a lot of references to design issues with the
SparseArrays package (https://github.com/JuliaLang/julia/issues/2326 /
https://github.com/JuliaLang/julia/pull/20815), which result in a
non-sensical dispatch arrangement, and contribute to a slow loading
experience do to the nonsense Unions that must be checked by subtyping.
