Smart reader (for `add` operation)

```js
/** 
 * @type {(read: (size: number) => Promise<UInt8Array>) => 
 *  (size: number) =>
 *  () => Promise<UInt8Array>
 * } 
 */
const reader = read => size => {
  let next = read(size)
  return async () => {
     let current = next
     next = current.then(() => read(size))
     return current
  }
}
```