stream: proper `instanceof` for `Writable`s

Use `[Symbol.hasInstance]()` to return `true` when asking for
`new Duplex() instanceof Writable`.

PR-URL: https://github.com/nodejs/node/pull/8834


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Calvin Metcalf <calvin.metcalf@gmail.com>