stream: add custom inspect to BufferList

Currently inspecting the BufferList can result a maximum call stack
size error. Adding a individual inspect function prevents this.

PR-URL: https://github.com/nodejs/node/pull/17907
Refs: https://github.com/nodejs/node/issues/12693


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>