Similarly to what Pimple did in v2 this PR makes declaring entries using closures simpler.

Before:

return [
    'foo' => DI\factory(function () {
        return new Foo();
    }),
];

After:

return [
    'foo' => function () {
        return new Foo();
    },
];

DI\factory() is now optional, closures will be automatically casted to factory definitions.

This only works with closures, DI\factory() still needs to be used with other callable types, e.g. invokable object, ['Doctrine\ORM\EntityManager', 'create'], …

If one wants to inject a callable (i.e. not execute it), DI\value() needs to be used:

return [
    'foo' => DI\value(function () {
        return new Foo();
    }),
    'bar' => DI\object('stdClass')
        ->constructor(DI\link('foo')),
];

This is more complex, but I don't think it's a problem as I've never ever seen that been used (same argument was used in Pimple).

@tsteur, @diosmosis thoughts?