Implement granular observability for collection elements

[nadako]

Basically haxetink/tink_state#49 (which is also implemented in haxetink/tink_state#74, so we can adapt that).

The idea is that if an Observable.Auto/AutoRun is only accessing a specific key from an observable list or dictionary, the recomputation should trigger only if that specific key is changed, not when anything is changed inside the collection.

Example:

var dict = Observable.Dictionary<string, int>();
dict["a"] = 1;
dict["b"] = 1;

Observable.AutoRun(() => Console.WriteLine(dict["a"]));

dict["a"] = 2; // should trigger re-run
dict["b"] = 2; // should NOT trigger re-run