feat: generic relations

[canvural]

This PR mainly does 2 things:

• Before, to get the related model of a relation we were executing the users' code and then getting the related model with getRelatedModel method. While analyzing some public Laravel apps with Larastan I saw this caused some issues. So this PR changes the behavior and tries to find the related model statically.
• Relations are now also generic. This allows us to drop some extensions, and use stubs to provide the same functionality. Also, it'll make easier to fix any problem that can come up. We just need to edit the stubs.

The existing code is not affected by any changes. So this is not a breaking change.

[nunomaduro]

@canvural Would be cool if we could find another contributor that is using Larastan that can actually validate this pull requests. I don't work within this codebase for a while. So it's difficult for me to actually check if everything sounds good.

What do you think? Do you know someone? We can always ask on twitter.

[canvural]

That's a good idea! Sometimes I not sure if a particular thing that I'm doing is the right way or there is an easier way.

Unfortunately I don't know anyone. We had some contributors from time to time. But not sure if they are still interested or not.

So yeah we can try to ask on Twitter!