I thought I was being clever, but too clever = stupid.
The dummy code designed to flag when our MSRV is bumped and supports `NonNull::add` would fail to compile on Rust versions below 1.80.0. Precisely because of the condition this code is testing - it's not supported!
Fix this by skipping compilation unless running clippy.
Remove `SparseStack::is_empty` method. It's pointless as the stack is never empty.
Add a dummy `NonEmptyStack::is_empty` method that always returns `false`. This is also pointless, but it overrides `slice::is_empty` which is otherwise accessible via `Deref`.
