Use Date.now() for timeout checks

`Date.now()` is significantly faster than `new Date()`
(http://jsperf.com/new-date-vs-date-now-vs-performance-now/28)

Author: LeszekSwirski

src/compile.js

@@ -267,17 +267,20 @@ Compiler.prototype._gr = function (hint, rest) {
  */
 Compiler.prototype.outputInterruptTest = function () { // Added by RNL
     var output = "";
-    if (Sk.execLimit !== null) {
-        output += "if (new Date() - Sk.execStart > Sk.execLimit) {throw new Sk.builtin.TimeLimitError(Sk.timeoutMsg())}";
-    }
-    if (Sk.yieldLimit !== null && this.u.canSuspend) {
-        output += "if (new Date() - Sk.lastYield > Sk.yieldLimit) {";
-        output += "var $susp = $saveSuspension({data: {type: 'Sk.yield'}, resume: function() {}}, '"+this.filename+"',currLineNo,currColNo);";
-        output += "$susp.$blk = $blk;";
-        output += "$susp.optional = true;";
-        output += "return $susp;";
-        output += "}";
-        this.u.doesSuspend = true;
+    if (Sk.execLimit !== null || Sk.yieldLimit !== null && this.u.canSuspend) {
+            output += "var $dateNow = Date.now();";
+        if (Sk.execLimit !== null) {
+            output += "if ($dateNow - Sk.execStart > Sk.execLimit) {throw new Sk.builtin.TimeLimitError(Sk.timeoutMsg())}";
+        }
+        if (Sk.yieldLimit !== null && this.u.canSuspend) {
+            output += "if ($dateNow - Sk.lastYield > Sk.yieldLimit) {";
+            output += "var $susp = $saveSuspension({data: {type: 'Sk.yield'}, resume: function() {}}, '"+this.filename+"',currLineNo,currColNo);";
+            output += "$susp.$blk = $blk;";
+            output += "$susp.optional = true;";
+            output += "return $susp;";
+            output += "}";
+            this.u.doesSuspend = true;
+        }
     }
     return output;
 };
@@ -916,7 +919,7 @@ Compiler.prototype.outputSuspensionHelpers = function (unit) {
     var output = "var $wakeFromSuspension = function() {" +
                     "var susp = "+unit.scopename+".wakingSuspension; delete "+unit.scopename+".wakingSuspension;" +
                     "$blk=susp.$blk; $loc=susp.$loc; $gbl=susp.$gbl; $exc=susp.$exc; $err=susp.$err;" +
-                    "currLineNo=susp.lineno; currColNo=susp.colno; Sk.lastYield=new Date();" +
+                    "currLineNo=susp.lineno; currColNo=susp.colno; Sk.lastYield=Date.now();" +
                     (hasCell?"$cell=susp.$cell;":"");
 
     for (i = 0; i < localsToSave.length; i++) {
@@ -1502,10 +1505,10 @@ Compiler.prototype.buildcodeobj = function (n, coname, decorator_list, args, cal
     // all function invocations in call
     this.u.varDeclsCode += "var $blk=" + entryBlock + ",$exc=[],$loc=" + locals + cells + ",$gbl=this,$err=undefined,$ret=undefined,currLineNo=undefined,currColNo=undefined;";
     if (Sk.execLimit !== null) {
-        this.u.varDeclsCode += "if (typeof Sk.execStart === 'undefined') {Sk.execStart = new Date()}";
+        this.u.varDeclsCode += "if (typeof Sk.execStart === 'undefined') {Sk.execStart = Date.now()}";
     }
     if (Sk.yieldLimit !== null && this.u.canSuspend) {
-        this.u.varDeclsCode += "if (typeof Sk.lastYield === 'undefined') {Sk.lastYield = new Date()}";
+        this.u.varDeclsCode += "if (typeof Sk.lastYield === 'undefined') {Sk.lastYield = Date.now()}";
     }
 
     //
@@ -1819,10 +1822,10 @@ Compiler.prototype.cclass = function (s) {
     this.u.switchCode += "return(function $" + s.name.v + "$_closure(){";
     this.u.switchCode += "var $blk=" + entryBlock + ",$exc=[],$ret=undefined,currLineNo=undefined,currColNo=undefined;"
     if (Sk.execLimit !== null) {
-        this.u.switchCode += "if (typeof Sk.execStart === 'undefined') {Sk.execStart = new Date()}";
+        this.u.switchCode += "if (typeof Sk.execStart === 'undefined') {Sk.execStart = Date.now()}";
     }
     if (Sk.yieldLimit !== null && this.u.canSuspend) {
-        this.u.switchCode += "if (typeof Sk.lastYield === 'undefined') {Sk.lastYield = new Date()}";
+        this.u.switchCode += "if (typeof Sk.lastYield === 'undefined') {Sk.lastYield = Date.now()}";
     }
     this.u.switchCode += "while(true){";
     this.u.switchCode += this.outputInterruptTest();
@@ -2219,10 +2222,10 @@ Compiler.prototype.cmod = function (mod) {
     this.u.prefixCode = "var " + modf + "=(function($modname){";
     this.u.varDeclsCode = "var $gbl = {}, $blk=" + entryBlock + ",$exc=[],$loc=$gbl,$err=undefined;$gbl.__name__=$modname,$ret=undefined,currLineNo=undefined,currColNo=undefined;";
     if (Sk.execLimit !== null) {
-        this.u.varDeclsCode += "if (typeof Sk.execStart === 'undefined') {Sk.execStart = new Date()}";
+        this.u.varDeclsCode += "if (typeof Sk.execStart === 'undefined') {Sk.execStart = Date.now()}";
     }
     if (Sk.yieldLimit !== null && this.u.canSuspend) {
-        this.u.varDeclsCode += "if (typeof Sk.lastYield === 'undefined') {Sk.lastYield = new Date()}";
+        this.u.varDeclsCode += "if (typeof Sk.lastYield === 'undefined') {Sk.lastYield = Date.now()}";
     }
 
     this.u.varDeclsCode += "try {";