Fix stores with vec index method and expose opaque writer types

[ealmloff]

This PR addresses two pain points with stores:

1. There is a bug with the index method on Store<Vec<T>>. The rest of the methods use the index directly as the selector index, but the .index method instead hashes the index. This can cause some items to not update correclty
2. If you try to expose the index method through a wrapper method, you can't name the type returned from the index method. We have a concrete type already, but it wasn't re-exported. You need to name this type for the automatic Store<T, _> to Store<T> conversion to work on the component boundary