RFC: validate slice size against cache line size to limit false-sharing

[abonander]

The current implementation does not check for the possibility of false-sharing, which is an unexpected bottleneck in parallel computation.

It might be a good feature to have additional construction method which errors if slices meant for two different threads might end up sharing a cache line. Unfortunately the alignment of the allocation which Vec takes isn't controllable at the moment, so this can't be a 100% guarantee AFAIK.