### Basic LRU Map Usage with Length Limit

Source: https://github.com/koute/schnellru/blob/master/README.md

Demonstrates creating an LRU map with a fixed length capacity, inserting elements, accessing them to change their order, and observing automatic eviction when the capacity is exceeded.

```rust
use schnellru::{LruMap, ByLength};
let mut map = LruMap::new(ByLength::new(3));

// Insert three elements.
map.insert(1, "one");
map.insert(2, "two");
map.insert(3, "three");
assert_eq!(map.len(), 3);

// They're ordered according to which one was inserted last.
let mut iter = map.iter();
assert_eq!(iter.next().unwrap(), (&3, &"three"));
assert_eq!(iter.next().unwrap(), (&2, &"two"));
assert_eq!(iter.next().unwrap(), (&1, &"one"));

// Access the least recently inserted one.
assert_eq!(*map.get(&1).unwrap(), "one");

// Now the order's changed.
// The element we've accessed was moved to the front.
let mut iter = map.iter();
assert_eq!(iter.next().unwrap(), (&1, &"one"));
assert_eq!(iter.next().unwrap(), (&3, &"three"));
assert_eq!(iter.next().unwrap(), (&2, &"two"));

// Insert a fourth element.
// This will automatically pop the least recently accessed one.
map.insert(4, "four");

// Still the same number of elements.
assert_eq!(map.len(), 3);

// And this is the one which was removed.
assert!(map.peek(&2).is_none());

// And here's the new order.
let mut iter = map.iter();
assert_eq!(iter.next().unwrap(), (&4, &"four"));
assert_eq!(iter.next().unwrap(), (&1, &"one"));
assert_eq!(iter.next().unwrap(), (&3, &"three"));

```

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