Supporting vtables for closures

[Sam-Ni]

This PR adds support for vtable instantiation for closures.
The implementation involves two main steps:

1. Matching a closure implementation

2. Instantiating the corresponding vtable

Step 1 is implemented in translate_body.rs, while Step 2 is implemented in translate_trait_objects.rs.
The modifications in each file are described below.

1. translate_body.rs

• translate_rvalue
  Supports registering closure trait implementations. For example, in the test file dyn-fn.rs, the closure |counter| { ... } is matched in the hax::ImplExprAtom::Builtin case and registered as a VTableInstance.

2. translate_trait_objects.rs

• add_method_to_vtable_def
  Adds additional checks for FnOnce when constructing the method field in the FnOnce vtable, due to the incompatibility of FnOnce::call_once with dyn-compatibility.

• get_vtable_instance_info
  Supports retrieving the vtable struct reference and the corresponding trait declaration reference for closures, based on their kind (Fn, FnMut, or FnOnce).

• add_supertraits_to_vtable_value
  Supports filling vtable fields with built-in supertraits. For closures supertraits, they are translated as VTableInstance with TraitImplSource::Closure(kind), generating a vtable according to the closure kind. For other built-in supertraits, they are translated as VTableInstance with TraitImplSource::Normal, whose trait reference itself is used as the impl reference (e.g. @2 := &core::marker::MetaSized::{vtable}).

• gen_vtable_instance_init_body
  Supports generating the vtable body for closures. Closure implementations are treated as trait implementations with no associated items and a single specialized closure method (call or call_mut). call_once is ignored due to the incompatibility mentioned above. Based on this design, the function retrieves (trait_pred, items) for closures and uses mk_field to add the specialized closure method.

• generate_closure_method_shim_ref
  Implements the generation of the closure method shim reference used in the mk_field in gen_vtable_instance_init_body.