Implement resource handle management

[gfx-rs]

Implement resource handle management

[brendanzab]

This sounds cool!

[photex]

Just so I'm sure, Manager here is the analogue to EngineData in my prototype?

@kvark do you recall off the top of your head your suggestion for your suggested improvement to the Handle again? (I sorta wish gitter had search so I could more easily locate that bit of conversation)

[kvark]

@photex Yeah, I propose the name handle::Manager instead of EngineData.

My suggestion was to have the highest generation reserved for a "zombie" generation. In the remove() function we could first increment the generation, then only do swap_remove() if the current generation hasn't reached the zombie state.