Implement a weak `HashMap` based on `Ephemeron`s

[raskad]

We need an internal weak HashMap type that is integrated with our GC. With #2586 we are merging an initial version that uses HashMap<WeakGc<K>, V>. We want to optimize this by basing our WeakMap implementation on Ephemerons. To do this we need to implement or own HashMap type.

For more information see the discussion in #2586