Resolve/instantiate factories using the container

[mnapoli]

Fix #179, start of #197

return [
    'Foo' => DI\factory(['FooFactory', 'create']),
];

With this configuration, FooFactory will be created by the container.

If FooFactory::create() is static, no instance will be created (and the method will be called statically).

Performance tests and profiling showed a performance regression of up to +40% (worst case) to resolve factories (after working on optimizations). I think this is acceptable given it's only a small part of the whole container (the total performance regression should be negligible, especially since 5.1 already brings good improvements to compensate).

TODO:

• documentation
• changelog