[1.x] Adds support for first class callable syntax

[nunomaduro]

This pull request adds support for first class callable syntax.

Because the first class callable syntax result does not "look" like a regular closure when serialised, this pull request attempts to transform the result of the code to a regular closure. Let me show you some examples:

// Assuming...
class Person {
    public function getName1() {}
    final public function getName2() {}
    public static function getName3() {}
}

// Assuming the following syntax, used within the Person class...
$1 = $this->getName1(...);
$2 = $this->getName2(...);
$2 = $this->getName3(...);

// The variables `$1`, `$2` and `$3`, when serialised / deserialised:
$1 = function () {} // bound to Person instance
$2 = function () {} // bound to Person instance
$3 = static function () {} // statically bound to Person instance

Fixes #32.