Adds Javascript Support

Summary:
- As mentioned in the title, this PR adds Javascript support to Yoga. Two different builds are included in this PR thanks to [nbind](https://github.com/charto/nbind), which conveniently allow to target both Node.js' native addons and browser environments via asmjs with approximately the same codebase. That should solve #215.

- All tests successfully pass on both codepaths. You can run `yarn test:all` inside the `javascript` directory to test it.

- Because of a bug in nbind, the [following PR](https://github.com/charto/nbind/pull/57) needs to be merged and a new version released before this one can be safely merged as well.

- I had to use `double` types instead of `float` in the C++ bindings because of an Emscripten [bug](https://github.com/kripken/emscripten/issues/3592) where symbols aren't correctly exported when using floats.

- There's some tweaks to do before this PR is 100% ready to merge, but I wanted to have your opinion first. What do you think of this?

 ---

To do:

- [x] Ensure th
Closes https://github.com/facebook/yoga/pull/304

Reviewed By: mikearmstrong001

Differential Revision: D4375187

Pulled By: emilsjolander

fbshipit-source-id: 47248558a9506b7c512b5ef281cd12fe1a60cab7

javascript/tests/Benchmarks/YGBenchmark.js

@@ -0,0 +1,113 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+var Yoga = Yoga || require("../../sources/entry-" + process.env.TEST_ENTRY);
+
+var ITERATIONS = 2000;
+
+YGBENCHMARK("Stack with flex", function() {
+  var root = Yoga.Node.create();
+  root.setWidth(100);
+  root.setHeight(100);
+
+  var measureCounter = getMeasureCounter(Yoga);
+
+  for (var i = 0; i < ITERATIONS; i++) {
+    const child = Yoga.Node.create();
+    child.setMeasureFunc(measureCounter.inc);
+    child.setFlex(1);
+    root.insertChild(child, 0);
+  }
+
+  root.calculateLayout(Yoga.UNDEFINED, Yoga.UNDEFINED, Yoga.DIRECTION_LTR);
+  root.freeRecursive();
+});
+
+YGBENCHMARK("Align stretch in undefined axis", function() {
+  var root = Yoga.Node.create();
+
+  var measureCounter = getMeasureCounter(Yoga);
+
+  for (var i = 0; i < ITERATIONS; i++) {
+    var child = Yoga.Node.create();
+    child.setMeasureFunc(measureCounter.inc);
+    child.setHeight(20);
+    root.insertChild(child, 0);
+  }
+
+  root.calculateLayout(Yoga.UNDEFINED, Yoga.UNDEFINED, Yoga.DIRECTION_LTR);
+  root.freeRecursive();
+});
+
+YGBENCHMARK("Nested flex", function() {
+  var root = Yoga.Node.create();
+
+  var measureCounter = getMeasureCounter(Yoga);
+
+  var iterations = Math.pow(ITERATIONS, 1 / 2);
+
+  for (var i = 0; i < iterations; i++) {
+    var child = Yoga.Node.create();
+    child.setFlex(1);
+    root.insertChild(child, 0);
+
+    for (var ii = 0; ii < iterations; ii++) {
+      var grandChild = Yoga.Node.create();
+      grandChild.setMeasureFunc(measureCounter.inc);
+      grandChild.setFlex(1);
+      child.insertChild(grandChild, 0);
+    }
+  }
+
+  root.calculateLayout(Yoga.UNDEFINED, Yoga.UNDEFINED, Yoga.DIRECTION_LTR);
+  root.freeRecursive();
+});
+
+YGBENCHMARK("Huge nested layout", function() {
+  var root = Yoga.Node.create();
+
+  var iterations = Math.pow(ITERATIONS, 1 / 4);
+
+  for (var i = 0; i < iterations; i++) {
+    var child = Yoga.Node.create();
+    child.setFlexGrow(1);
+    child.setWidth(10);
+    child.setHeight(10);
+    root.insertChild(child, 0);
+
+    for (var ii = 0; ii < iterations; ii++) {
+      var grandChild = Yoga.Node.create();
+      grandChild.setFlexDirection(Yoga.FLEX_DIRECTION_ROW);
+      grandChild.setFlexGrow(1);
+      grandChild.setWidth(10);
+      grandChild.setHeight(10);
+      child.insertChild(grandChild, 0);
+
+      for (var iii = 0; iii < iterations; iii++) {
+        var grandGrandChild = Yoga.Node.create();
+        grandGrandChild.setFlexGrow(1);
+        grandGrandChild.setWidth(10);
+        grandGrandChild.setHeight(10);
+        grandChild.insertChild(grandGrandChild, 0);
+
+        for (var iiii = 0; iiii < iterations; iiii++) {
+          var grandGrandGrandChild = Yoga.Node.create();
+          grandGrandGrandChild.setFlexDirection(Yoga.FLEX_DIRECTION_ROW);
+          grandGrandGrandChild.setFlexGrow(1);
+          grandGrandGrandChild.setWidth(10);
+          grandGrandGrandChild.setHeight(10);
+          grandGrandChild.insertChild(grandGrandGrandChild, 0);
+        }
+      }
+    }
+  }
+
+  root.calculateLayout(Yoga.UNDEFINED, Yoga.UNDEFINED, Yoga.DIRECTION_LTR);
+  root.freeRecursive();
+});