Closes #20

Idea: we don't need to create a custom API to manage items in Dialog::$memory, Collection implementations (e.g. Laravel Collection) already provide a way for this. We just need to make sure that existing serialised dialogs work with this new implementation.

New code example:

$this->memory->put($key, $value);
$this->memory->forget($key);
$value = $this->memory->get($key, 'defalut');
$value = $this->memory->pull($key);

Old code example:

$this->remember($key, $value);
$this->forget($key);
$value = $this->memory[$key] ?? 'defalut';

Because of these changes, we don't need Dialog::remember and Dialog::forget methods anymore. I market them as deprecated and they will be removed in the v1.0