*
 * This collection is optimized for safe concurrent access over a very small number of elements.
 *
 * This collection and is expected to hold fewer than 10 elements. Each operation is _O(N)_, and so
 * building an instance with _N_ elements is _O(N**2)_.
 */
internal sealed class Tags {
  /**
   * Returns a tags instance that maps [key] to [value]. If [value] is null, this returns a tags