Replace uses of `ptr::Unique` and `ptr::Shared` with `ptr::NonNull`

[hawkw]

These types were renamed recently in the standard lib. For context, see also sos-os/alarm#12, sos-os/alarm#25.

[hawkw]

This should be more or less trivial; just a find and replace. Will fix the build, though, so that's big!

[hawkw]

The #![feature(ptr_internals)] flags added in 847c308 and c84ca3b should also be removed when fixing this.

[hawkw]

No longer critical or bug as this no longer breaks the build after 847c308 and c84ca3b, but referring to this type by its old name is still deprecated & should be changed.