[ExpressionLanguage] Added closures support

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/11938
| License       | MIT
| Doc PR        | 

This PR adds closures support to the `ExpressionLanguage` for both compiled and evaluated modes.

As @stof suggested in https://github.com/symfony/symfony/issues/11938, I added a new syntax to differentiate them from functions. The simplest syntax I could come up with is to prepend a `$` to a function name.

## Evaluation mode

In evaluation mode, you can pass the closure as a variable when evaluating the expression.

```php
// => '3'
echo $language->evaluate('$add(1, 2)', array(
    'add' => function ($a, $b) {
        return $a + $b;
    }
));
```

## Compilation mode

When compiled, the closure expression is unchanged because it uses the same syntax than php .

```php
$hello = function () {
    return 'hello';
};

$world = function () {
    return 'world';
};

$language = new ExpressionLanguage();

// => hello world
eval(sprintf('echo %s;', $language->compile('$hello() ~ " " ~ $world()', array('hello', 'world'))));
```