streamExtend flattens (iterates over values)

[richytong]

currently does not flatten

Update transform docs as well to this

 * If the semigroup is an object with a `.write` method, concatenation is defined as:
 * ```javascript
 * nextAccumulator = accumulator
 * for (const value of values) {
 *   accumulator.write(value)
 * }
 * ```

Update: flatten could be implicit