`memhash` is to be deleted in 1.13

[adienes]

https://github.com/JuliaData/CSV.jl/blob/04ec1cf3a7b0f069c573ba7c9da351026427532b/src/utils.jl#L581

this line defines hash on PointerString using memhash, which will be removed in 1.13

note that there are some in-progress plans to simplify the migration path but I just wanted to give advance warning