buffer: optimize Buffer#toString()

Break up Buffer#toString() into a fast and slow path.  The fast path
optimizes for zero-length buffers and no-arg method invocation.

The speedup for zero-length buffers is a satisfying 700%.  The no-arg
toString() operation gets faster by about 13% for a one-byte buffer.

This change exploits the fact that most Buffer#toString() calls are
plain no-arg method calls.  Rewriting the method to take no arguments
means a call doesn't go through an ArgumentsAdaptorTrampoline stack
frame in the common case.

PR-URL: https://github.com/nodejs/io.js/pull/2027


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Christian Tellnes <christian@tellnes.no>
Reviewed-By: Daniel Cousens <email@dcousens.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>