Support taking promises for the initial state

```
useCart = functioin(){

useAsyncReducer( getCart() ,{

})
}
```

## Considerations

- What would `state` be while the initialState promise is loading?

## How to make this work now

```
useCart = functioin(){

[action] =useAsyncReducer( null ,{
getCart(){}
})
action.getCart()
}
```

The issue names useAsyncReducer and getCart but no files or tests. Start by locating useAsyncReducer and tracing how its initial state is consumed; define the loading-state behavior for a promise initial state, then verify that promise initialization and its resulting state are covered.

Written by the indexing model from the issue text.