Macro dispatch_types

macro_rules! dispatch_types {
    (
        ref type $name:ident: $interface:ident = $core_type:ident,$webgpu_type:ident,$custom_type:ident
    ) => { ... };
    (
        mut type $name:ident: $interface:ident = $core_type:ident,$webgpu_type:ident,$custom_type:ident
    ) => { ... };
}

Generates Dispatch types for each of the interfaces. Each type is a wrapper around the wgpu_core and webgpu types, and derefs to the appropriate interface trait-object.

When there is only one backend, devirtualization fires and all dispatches should turn into direct calls. If there are multiple, some dispatching will occur.

This also provides as_* methods so that the backend implementations can dereference other arguments. These are similarly free when there is only one backend.

In the future, we may want a truly generic backend, which could be extended from this enum.