stream: avoid additional validation for Buffers

These changes result in ~50% improvement in the included benchmark.

PR-URL: https://github.com/nodejs/node/pull/10580


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>