events: optimize eventEmitter.once()

This basically removes the extra closure previously created by
once() and instead uses an object to denote a one-time handler.
The result is ~340% performance increase when adding one-time
handlers, ~48% performance increase when emitting events with
mixed persistent and one-time handlers, and ~10% performance
increase when emitting with multiple arguments with mixed handlers,
all with negligible negative performance impact to other EventEmitter
functionality.

Also, removeListener() was optimized a bit by eliminating handler
arrays of length 1 so that the leftover handler is set on
_events[type].

lib/_stream_readable.js

@@ -543,10 +543,16 @@ Readable.prototype.pipe = function(dest, pipeOpts) {
   // is attached before any userland ones.  NEVER DO THIS.
   if (!dest._events || !dest._events.error)
     dest.on('error', onerror);
-  else if (Array.isArray(dest._events.error))
-    dest._events.error.unshift(onerror);
-  else
-    dest._events.error = [onerror, dest._events.error];
+  else {
+    var existingHandler = dest._events.error;
+    if (Array.isArray(existingHandler))
+      existingHandler.unshift(onerror);
+    else {
+      var handlers = [onerror, existingHandler];
+      handlers.onceCount = (existingHandler.once ? 1 : 0);
+      dest._events.error = handlers;
+    }
+  }
 
 
 
@@ -651,9 +657,7 @@ Readable.prototype.unpipe = function(dest) {
 
 // set up data events if they are asked for
 // Ensure readable listeners eventually get something
-Readable.prototype.on = function(ev, fn) {
-  var res = Stream.prototype.on.call(this, ev, fn);
-
+Readable.prototype._on = function(ev) {
   // If listening to data, and it has not explicitly been paused,
   // then call resume to start the flow of data on the next tick.
   if (ev === 'data' && false !== this._readableState.flowing) {
@@ -677,7 +681,15 @@ Readable.prototype.on = function(ev, fn) {
       }
     }
   }
-
+};
+Readable.prototype.on = function(ev, fn) {
+  var res = Stream.prototype.on.call(this, ev, fn);
+  this._on(ev);
+  return res;
+};
+Readable.prototype.once = function(ev, fn) {
+  var res = Stream.prototype.once.call(this, ev, fn);
+  this._on(ev);
   return res;
 };
 Readable.prototype.addListener = Readable.prototype.on;