fix(cordyceps): fix use-after-free in List iterators

[hawkw]

The cordyceps::List Iter and Cursor types currently have incorrect
Item types that can result in a UAF if used incorrectly. These
iterators return T::Handle as their Item types...but a Handle
owns an element, and dropping the handle can drop the element. If, for
example, the element is Box, then dropping the Handle will
deallocate the memory that the entry lives in. Similarly, dropping an
Arc entry would perform an invalid reference count decrement.

If an iterator over a list is created twice, and the list contains a
handle type such as Box<T>, then the second iterator will cause a
double free. More worryingly, accessing any entries from the list will
cause a use-after-free.

We missed this issue since

1. there weren't any tests for the iterators, and
2. all the tests currently use &'a Entry as the handle type, so
   dropping an entry doesn't free memory.

This branch fixes the use-after-free by changing the Iter and Cursor
types to return references to the element, rather than Handles. We
will add an additional Drain iterator that actually removes elements
from the list and returns the Handle type, in a follow-up PR.

I've added a test which fails against the current main branch.

BREAKING CHANGE:

This changes the type signature of the list::Iter and list::Cursor
types.