optimize Sequence for performance by using Promise.each

Previously, Sequence was checking for Promise-type result
every time, to detect when a Promise needs to be returned
from .run() method, which created a deep Promise chain.

After this change, when Sequence finds Promise-type result
it uses Promise.each on all the remaining tests.

lib/sequence.js

@@ -1,5 +1,8 @@
 'use strict';
 var isPromise = require('is-promise');
+var AvaError = require('./ava-error');
+
+function noop() {}
 
 module.exports = Sequence;
 
@@ -17,21 +20,17 @@ function Sequence(tests, bail) {
 	this.reason = null;
 	this.tests = tests;
 	this.bail = bail || false;
+
+	// TODO(vdemedes): separate into a utility (it's being used in serveral places)
+	Object.keys(Sequence.prototype).forEach(function (key) {
+		this[key] = this[key].bind(this);
+	}, this);
 }
 
 Sequence.prototype.run = function () {
-	return this._run();
-};
-
-// run sequence items starting from specified index
-Sequence.prototype._run = function (fromIndex) {
 	var length = this.tests.length;
 
-	if (!fromIndex) {
-		fromIndex = 0;
-	}
-
-	for (var i = fromIndex; i < length; i++) {
+	for (var i = 0; i < length; i++) {
 		// if last item failed and we should bail, return results and stop
 		if (this.bail && !this.passed) {
 			return this._results();
@@ -39,26 +38,41 @@ Sequence.prototype._run = function (fromIndex) {
 
 		var result = this.tests[i].run();
 
+		// if a Promise returned, we don't need to check for Promises after this test
+		// so we can just use Promise.each() on the rest of the tests
 		if (isPromise(result)) {
-			return this._awaitResult(result, i + 1);
+			return result
+				.then(this._addResult)
+				.return(this.tests.slice(i + 1))
+				.each(this._runTest)
+				.catch(AvaError, noop)
+				.then(this._results);
 		}
 
+		try {
 			this._addResult(result);
+		} catch (err) {
+			// in bail mode, don't execute the next tests
+			if (err instanceof AvaError) {
+				return this._results();
+			}
+
+			throw err;
+		}
 	}
 
 	return this._results();
 };
 
-// if result is a Promise, return it to make Sequence thennable
-// add resolved result and continue execution from the next item
-Sequence.prototype._awaitResult = function (result, nextIndex) {
-	var self = this;
+Sequence.prototype._runTest = function (test) {
+	var result = test.run();
 
-	return result.then(function (ret) {
-		self._addResult(ret);
+	if (isPromise(result)) {
+		return result
+			.then(this._addResult);
+	}
 
-		return self._run(nextIndex);
-	});
+	return this._addResult(result);
 };
 
 Sequence.prototype._addResult = function (result) {
@@ -71,7 +85,13 @@ Sequence.prototype._addResult = function (result) {
 		if (!this.reason) {
 			this.reason = result.reason;
 		}
+
+		if (this.bail) {
+			throw new AvaError('Error in Sequence while in bail mode');
+		}
 	}
+
+	return result;
 };
 
 Sequence.prototype._results = function () {