Add type-erased wrappers around Specializer and SpecializerKey

Currently, type-erased materials have their key type and key getter located in AsBindGroup, and their specialize fn located in Material. This is mostly an experiment, but I think erased specializers might be a good way to wrap all that logic up nicely.

Vaguely what I'm imagining:

trait AsBindGroup {
-  type Data;
-  fn bind_group_data(&self);
}

trait Material {
-  fn specialize(...);
+  fn specializer() -> MaterialSpecializer {
+    MaterialSpecializer::new(
+      MySpecializer,
+      |material, mesh_key| MyKey(...),
+  }
}

Currently there's more boxing than there should be, I'll probably switch to some form of bytemuck or SmallBox later