Make `kani::mem::is_inbounds` public

[jswrenn]

In zerocopy, we have a constructor whose safety condition is that its argument is a pointer that is well-sized for its type and metadata. I see that can_write_unaligned is currently just a wrapper around in_bounds, but our safety condition doesn't have anything to do with writeability.

[rajath-mk]

Hi, Thanks for raising this issue! We've made kani::mem::is_inbounds public to directly address your use case. Please see PR #3958 for the changes.