Generator use try stack runtime

By using a stack to keep track of try/catch/finally we can move a lot of logic from the generated code to a runtime function. This allows the codegen to become significantly smaller.

```js
function* f() {
  try {
    yield 1;
  } catch (ex) {
    yield ex;
  }
}
```

Before:
```js
function f() {
  var ex;
  var $that = this,
      $arguments = arguments,
      innerFunction = function($ctx) {
        while (true) switch ($ctx.state) {
          case 0:
            $ctx.state = 2;
            return 1;
          case 2:
            if ($ctx.action == 1) {
              $ctx.action = 0;
              throw $ctx.sent;
            }
            $ctx.state = 4;
            break;
          case 9:
            ex = $ctx.storedException;
            $ctx.state = 5;
            break;
          case 5:
            $ctx.state = 6;
            return ex;
          case 6:
            if ($ctx.action == 1) {
              $ctx.action = 0;
              throw $ctx.sent;
            }
            $ctx.state = 4;
            break;
          case 4:
            $ctx.state = -2;
          case -2:
            return $ctx;
          case -3:
            throw $ctx.storedException;
          default:
            throw "traceur compiler bug: invalid state in state machine: " + $ctx.state;
        }
      },
      moveNext = function($ctx) {
        while (true) {
          try {
            return innerFunction($ctx);
          } catch (ex) {
            $ctx.storedException = ex;
            switch ($ctx.state) {
              case 2:
                $ctx.state = 9;
                break;
              default:
                $ctx.GState = 3;
                $ctx.state = -2;
                throw $ctx.storedException;
            }
          }
        }
      };
  return $traceurRuntime.generatorWrap(moveNext);
}
```

After:
```js
function f() {
  var ex;
  var $that = this,
      $arguments = arguments;
  return $traceurRuntime.generatorWrap(function($ctx) {
    while (true) switch ($ctx.state) {
      case 0:
        $ctx.pushTry(9, null);
        $ctx.state = 12;
        break;
      case 12:
        $ctx.state = 2;
        return 1;
      case 2:
        if ($ctx.action == 1) {
          $ctx.action = 0;
          throw $ctx.sent;
        }
        $ctx.state = 4;
        break;
      case 4:
        $ctx.popTry();
        $ctx.state = 14;
        break;
      case 9:
        $ctx.popTry();
        ex = $ctx.storedException;
        $ctx.state = 5;
        break;
      case 5:
        $ctx.state = 6;
        return ex;
      case 6:
        if ($ctx.action == 1) {
          $ctx.action = 0;
          throw $ctx.sent;
        }
        $ctx.state = 14;
        break;
      case 14:
        $ctx.state = -2;
      case -2:
        return $ctx;
      case -3:
        throw $ctx.storedException;
      default:
        throw "traceur compiler bug: invalid state in state machine: " + $ctx.state;
    }
  });
}
```

Review URL: https://github.com/google/traceur-compiler/pull/699

Closes #699.