Automatic GC

I'd love to build an automatic GC for mst-reference-pool.

Right now, you have to pass in any node that might reference an instance in the pool. It's not the end of the world, but it is rather manual.

```ts
store.gc([
store.currentUser,
store.friends,
store.postStore.postAuthors
])
```

It would be cool if you could just have it traverse the tree itself and find any locations that might contain references itself. And probably cache that to make it fast.

```ts
store.gc()
```

The issue provides no file or test entry points; start by locating the existing store.gc implementation and the reference-pool traversal logic. Define how store.gc() should discover references without explicit nodes, how cached traversal stays current, and what tests demonstrate equivalent results to the current manual API.

Written by the indexing model from the issue text.