Move Arc inside dispatch enum

[sagudev]

With #6665 wgpu::Adapter become Arc<dispatch::DispatchAdapter>, but we could move Arc one level lower into Dispatch*, like so:

#[derive(Debug, Clone)]
pub struct Adapter {
    pub(crate) inner: DispatchAdapter,
}

pub enum DispatchAdapter {
    #[cfg(wgpu_core)]
    Core(Arc<CoreAdapter>),
    #[cfg(webgpu)]
    WebGPU(Arc<WebAdapter>),
}

This way PartialEq, Eq, PartialOrd, Ord, Hash can start depending on inner arcs, so there would be no need to bound them on InterfaceTypes. This will make #6658 simpler and more inline with this (and will also remove more Arc indirections).

This would allow us to eventually remove Arc wrapping Core as we already have Arcs inside Core types (essentially lowering Arc even further down).

cc @cwfitzgerald

[sagudev]

As noted in #6665 (comment) the only problems are &mut types (that are not clonable now anyway) so we need special handling of those (to keep old behavior). Here is how changes would look like: https://github.com/sagudev/wgpu/tree/arc-in-dispatch

SIDE-NOTE: On custom context Arc::get_mut seems good for those as only one instance allowed externally anyway.