Rename internal_object to handle

[Rua]

Changelog:

### Breaking changes
Changes to the `VulkanObject` trait:
- The method `internal_object` is renamed to `handle`, and the associated type `Object` is renamed to `Handle`.

This is something I've been wanting to do for a while. In Vulkan terminology these are "handles" and we already use that terminology elsewhere: the private variables holding them are called handle and we have from_handle constructors now too.

[AustinJ235]

Looks good, needs some conflicts addressed. You can go ahead with the merge when those are resolved.

[marc0246]

I love this too, has been bothering me for a while haha!