feat: Add the Tail higher-order stream

[Hywan]

This patch adds the Tail higher-order stream. Similarly to Head,
it computes a limited view of the underlying ObservableVector's
items, however the view starts from the end —instead of the start— of
the ObservableVector.

use eyeball_im::{ObservableVector, VectorDiff};
use eyeball_im_util::vector::VectorObserverExt;
use imbl::vector;
use stream_assert::{assert_closed, assert_next_eq, assert_pending};

// Our vector.
let mut ob = ObservableVector::<char>::new();
let (values, mut sub) = ob.subscribe().tail(3);

assert!(values.is_empty());
assert_pending!(sub);

// Append multiple values.
ob.append(vector!['a', 'b', 'c', 'd']);
// We get a `VectorDiff::Append` with the latest 3 values!
assert_next_eq!(sub, VectorDiff::Append { values: vector!['b', 'c', 'd'] });

// Let's recap what we have. `ob` is our `ObservableVector`,
// `sub` is the “limited view” of `ob`:
// | `ob`  | a b c d |
// | `sub` |   b c d |

// Append multiple other values.
ob.append(vector!['e', 'f']);
// We get three `VectorDiff`s!
assert_next_eq!(sub, VectorDiff::PopFront);
assert_next_eq!(sub, VectorDiff::PopFront);
assert_next_eq!(sub, VectorDiff::Append { values: vector!['e', 'f'] });

// Let's recap what we have:
// | `ob`  | a b c d e f |
// | `sub` |       d e f |
//             ^ ^   ^^^
//             | |   |
//             | |   added with `VectorDiff::Append { .. }`
//             | removed with `VectorDiff::PopFront`
//             removed with `VectorDiff::PopFront`

assert_pending!(sub);
drop(ob);
assert_closed!(sub);

---

• Built on top of chore: Rename Limit to Head #70, should be merged after

[Hywan]

Rebased on top of #70, and renamed RLimit to Tail

[Hywan]

Rebased on top of main (which now includes #70).