[Scoper] Callables scoping

[theofidry]

There is several callables that could be supported:

$x = ['Acme\Foo', 'bar'];
$x = 'Acme\Foo::bar';

The limitation being this would not work on a class from the global namespace since it would be hard to tell apart a class from a regular string.

To check however if this is worth it as it might cause more problems than it solves.

[theofidry]

Note that there is safe cases, for example spl_autoload_register (already handled) or set_error_handler (not handled at the time of writing)

[theofidry]

Closing as the remaining part is the same as #294.