Set the default type argument of Binder to Y.Snapshot

`Binder` is defined as follows:

```ts
export type Binder<S extends Snapshot> = ...
```

It would be nice if we would make the type argument optional:

```ts
export type Binder<S extends Snapshot = Snapshot> = ...
```

Then it would be consistent with `binder<T>()` which infers `T` as `Snapshot` if it's not explicitly provided. 