`memhash` is to be deleted in 1.13

[adienes]

https://github.com/chengchingwen/BytePairEncoding.jl/blob/a4ea37c604087e9d2973c17e80428d2f4fa32b07/src/mstring.jl#L97

seems that memhash is used for hash(::Merge), but this will be deleted in 1.13

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