Store trait APIs should use u64 instead of usize

[allada]

We currently use usize for the APIs in StoreLike, but they should be u64. It would be nice to move these all to use u64 instead. This is because u64 expresses the size of the data, but usize expresses local memory limits.

This will be a big PR, but mostly cosmetic with some as usize and as u64's and/or usize::try_from() and u64::try_from()'s added and removed.

[boldpulse]

I will solve this issue.

cc: @allada, @MarcusSorealheis