`Stack` is a stack structure, optimized for fast push/pop and reading/writing the last entry on the stack. The difference from `NonEmptyStack` is that it can be empty.
This has a different trade-off vs `NonEmptyStack`:
* `Stack::new` does not allocate (`NonEmptyStack` does).
* `Stack::last` and `Stack::last_mut` are fallible and contain a branch (those methods on `NonEmptyStack` are branchless and infallible).
`Stack` is only the better choice if:
1. You want `new()` not to allocate. or
2. Creating initial value for `NonEmptyStack::new()` is expensive.
Use `Stack` as one of the backing stores in `SparseStack`.
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