lib: micro-optimize EventEmitter#removeListener()

Replace the call to Array#splice() with a faster open-coded version
that creates less garbage.

Add a new benchmark to prove it.  With the change applied, it scores
a whopping 40% higher.

PR-URL: https://github.com/iojs/io.js/pull/185


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>