Add features for layer development

[Sorenon]

This makes two minor changes to make openxrs more practical for developing an OpenXR layer.

Adding Hash to raw handle types makes it simpler to manage all the tracked handles like so:

static mut INSTANCE_WRAPPERS: OnceCell<DashMap<xr::Instance, Arc<instance::InstanceWrapper>>> = OnceCell::new();

And the added bindings from loader_interfaces.h are used for Runtime-Loader-Layer negotiation.

[Sorenon]

Thanks! Would it make sense to generate these, rather than hand-writing them?

Since the file is so small and not defined by XML I think that writing a generator would be overkill.

[Ralith]

Ah, yeah, if there's no XML source then the effort isn't justified. Nice job keeping the style consistent!

[Ralith]

Sorry this languished; Actions bugged out somehow.