Introduce a factory to build proofs

[Baptouuuu]

This factory would be passed as argument to the callable passed in Application::tryToProve().

It would look like this:

Application::new([])
    ->tryToProve(static function($factory) {
        yield $factory->proof(
            'name',
            given($set),
            static fn($assert, $value) => null,
        );
    });

And existing functions should now be deprecated.