feature request: add .Clear method

Currently, it's impossible to clear the `OrderedMap` keeping its capacity.

A simple implementation would be:
```go
func (l *list[K, V]) Clear() {
l.root = Element[K, V]{} // just in case K & V are pointer types
}

func (m *OrderedMap[K, V]) Clear() {
clear(m.kv)
m.ll.Clear()
}
```