Add read helpers

[fu5ha]

Motivated by Traverse-Research/gpu-allocator#139, scaffolded some read-helper functions.

These helpers are mostly all unsafe and have both semi-checked and completely unchecked variants. They are relevant in either case because they lay out in documentation exactly what the needed safety requirements are to read the given data, given that we have a properly implemented Slab, and try to remove some common footguns (alignment, size within allocation) where possible in the checked variants.

A note for reviewers is that you can skip the copy.rs file as that is just code movement from the old lib.rs.

@eddyb, requested your review since you helped validate presser originally and you might have some valuable input on the safety comments/requirements here.

[fu5ha]

Thanks for the review @MarijnS95 , though, FWIW all your comments are in the copy module which is entirely unchanged in this PR 😅 haha. It's just code movement. The read module is the new stuff.