Simpler scopes: one less Composer dependency and better performances

[mnapoli]

The internal implementation of scopes has been simplified: this results in one less Composer dependency and better performances. Backward compatibility is kept (static methods still work).

Before:

return [
    'MyClass' => DI\object()
        ->scope(Scope::PROTOTYPE()), // static method
];

After:

return [
    'MyClass' => DI\object()
        ->scope(Scope::PROTOTYPE), // constant
];

This is more inline with PSR-1/PSR-2, more natural (the concept of enum was a bit forced here) and more efficient.