docs: add RFC for opaque generational handles

[gituser12981u2]

Added RFC for opaque handles.

Please put all comments about this RFC here.

[alexcu2718]

I do wonder if the singular handle type view maybe a blocker for more eccentric design choices down the road?

One thing we may want to do is use (or create a simple one ourselves?) for a bump allocator, it's piss easy if it's thread local to self write but obviously not suitable. Probably best to investigate this?

(The bump allocator would have to be non-homogenous, pretty simple though.)

*Maybe this comes into mind later down the road, when we're handling game entity objects.

Other than that random discourse, looks really slick. Also love the concept use haha.

[gituser12981u2]

Okay, I add allocator input (optional)