`memhash` is to be deleted in 1.13

[adienes]

https://github.com/sl-solution/InMemoryDatasets.jl/blob/b02dc59da12a92f41114b8cbee94b677f15f584f/src/characters/characters.jl#L137

it's used here; ref https://github.com/JuliaLang/julia/issues/59151

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