buffer: improve toJSON() performance

PR-URL: https://github.com/nodejs/node/pull/10895


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>