Mitigate unsoundness in InternalState

[Razican]

This comes from #363 (comment). It turns out that implementing an empty trace was not safe if the contained type was not Trace.

I'm still having issues, though, since we cannot downcast a non Any type. Adding Any to the InternalState trait bounds doesn't help either. Any idea on how to fix this, @HalidOdat or @jasonwilliams ?

This still has the issue that Regex does not implement Trace, and in the future it could, which would make it unsound again. In any case, I think this is a good compromise for now. In the future we will probably need to change the Regex for a different thing, in any case, since it doesn't implement everything in the JavaScript regular expressions.

[Razican]

It seems that this is not enough to be able to call downcast_mut() on self.state. It gives the following error:

method not found in `&(dyn builtins::object::internal_state::InternalState + 'static)`

[Razican]

After some refactoring, I'm getting this error:

error[E0605]: non-primitive cast: `&dyn builtins::object::internal_state::InternalState` as `&(dyn std::any::Any + 'static)`
  --> boa/src/builtins/object/internal_state.rs:44:9
   |
44 |         (state as &dyn Any).downcast_ref::<T>()
   |         ^^^^^^^^^^^^^^^^^^^
   |
   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait

error[E0605]: non-primitive cast: `&mut (dyn builtins::object::internal_state::InternalState + 'static)` as `&mut (dyn std::any::Any + 'static)`
  --> boa/src/builtins/object/internal_state.rs:51:9
   |
51 |         (state as &mut dyn Any).downcast_mut::<T>()
   |         ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait

[HalidOdat]

Superseded by #626