Ignore fat pointer vtables for Gc::ptr_eq

Rust doesn’t guarantee that fat pointer comparison works as expected
due to duplicated vtables:

rust-lang/rust#46139

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

gc/src/gc.rs

@@ -121,7 +121,7 @@ impl<T: Trace> GcBox<T> {
 impl<T: Trace + ?Sized> GcBox<T> {
     /// Returns `true` if the two references refer to the same `GcBox`.
     pub(crate) fn ptr_eq(this: &GcBox<T>, other: &GcBox<T>) -> bool {
-        // Ignore vtables with .header to work around
+        // Use .header to ignore fat pointer vtables, to work around
         // https://github.com/rust-lang/rust/issues/46139
         ptr::eq(&this.header, &other.header)
     }