buffer: optimize Buffer.byteLength

Buffer.byteLength is important for speed because it is called whenever a
new Buffer is created from a string.

This commit optimizes Buffer.byteLength execution by:
- moving base64 length calculation into JS-land, which is now much
  faster
- remove redundant code and streamline the UTF8 length calculation

It also adds a benchmark and better tests.

PR-URL: https://github.com/nodejs/io.js/pull/1713


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>