Add naive generic array implementation

[danog]

FIxes https://github.com/cweiske/jsonmapper/issues/243

[danog]

This is a very bare-bones, naive implementation that is intentionally kept simple as I saw https://github.com/cweiske/jsonmapper/issues/165.

Ideally, this should just bring phpstan's phpdoc parser or even valinor and re-use their logic, but this would require a major overhaul.

This PR, on the other hand, aims at a very simple and barebones implementation, changing as little as possible to avoid pouring time in a PR that might get refused.

[SvenRtbg]

I understand this approach, and the reasoning behind it.

It will lead to unmaintainable code in the long term, though, because everyone is scared to do more than just the required minimum if the invested work would be rejected.

[danog]

I'm sorry, but I don't have the bandwidth for a full implementation right now, I'm only upstreaming this patch as it is used by one of my clients.