Follow-up for light client extraction PR

[hu55a1n1]

Summary

This is the follow-up issue for PR informalsystems/hermes#2483.

• Use the downcast-rs crate - This will allow us to remove the AsAny and AsAnyMut code we currently have. It also provides a way for going from boxed trait objects to boxed concrete types without cloning, something that isn't possible with the current API. eg. Box<dyn ClientState> -> Box<TmClientState>.
• Use macros for ErasedPartialEq defs and impls. Ideally all erased traits and their blanket impls must follow the convention that most well-known dyn-* rust crates use, for e.g. dyn-clone ->

  /// Define trait with supertrait
  trait MyTrait: DynClone {
      /* ... */
  }
  
  /// Use proc-macro to provide default impl
  dyn_clone::clone_trait_object!(MyTrait);

• Separate trait for into_box(). (downcast-rs might already have a solution for this)

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