Create a class `PersistentToLocal` which contains three methods,
`Strong`, `Weak`, and `Default`:

* `Strong` returns a `Local` from a strong persistent reference,
* `Weak` returns a `Local` from a weak persistent reference, and
* `Default` decides based on `IsWeak()` which of the above two to call.

These replace `node::StrongPersistentToLocal()`,
`node::WeakPersistentToLocal()`, and `node::PersistentToLocal()`,
respectively.