Add Apecs.Experimental.Children module

[jship]

This PR adds Apecs.Experimental.Children to support one-to-many relationships. A "parent" entity may have multiple values of a particular component type. This is like how in apecs-physics an entity with a Body component value may have many collision Shape component values (where each Shape value is owned by another distinct entity), but general-purpose so that we can build this kind of relationship for any component types.

There is an example that demonstrates usage and is likely a good starting point to get a feel for what the interface is like.

I'm not totally sure this functionality belongs in the main apecs library, though I feel the internals are fiddly enough that it'd be nice saving others from reimplementing similar behavior. I'm finding the functionality useful in my projects, but I could also see the code being pushed into a new library if that's preferred.

[jonascarpay]

This is excellent, thank you for including tests and an example. I'd be happy to include this.

It took me a while to figure out where to look for the exact functionality that this provides, and why I'd be interested in using this module. Please move the synopsis to the top, include

• Concretely, what you get by using this module instead of just including a parent field in your component data type. In other words, highlight the ChildList and the functionality that it provides.
• one or more (very brief) example use-cases
• and then the link to the example

[jonascarpay]

Thanks again! Sorry for slow responses