Freeing memory once a derivation is no longer needed

Consider this code:

```
a = atom(5)
d = derivation(() => a.get() + 1)
d.get()
```

A side effect of `d.get()` is setting `a._children[0]` to point to `d`.

When I'm done with `d`, how do I get rid of that pointer?

Right now I use derivations in my React components which call `.get()` on derivables outside of the component. When the component is unmounted, the pointers like `a._children[0]` above are causing a major memory leak, because the entire component is sticking around as the closure-context to the un-garbage-collectable derivation.