Make the storage of wasmtime_runtime::Table consistent.

This change makes the storage of `Table` more internally consistent.

Elements are stored as raw pointers for both static and dynamic table storage.

Explicitly storing elements as pointers removes assumptions being made by the
pooling allocator in terms of the size and default representation of the
elements.

However, care must be made to properly clone externrefs for table operations.

crates/runtime/src/externref.rs

@@ -351,8 +351,35 @@ impl VMExternRef {
         ptr
     }
 
+    /// Consume this `VMExternRef` into a raw, untyped pointer.
+    ///
+    /// # Safety
+    ///
+    /// This method forgets self, so it is possible to create a leak of the
+    /// underlying reference counted data if not used carefully.
+    ///
+    /// Use `from_raw` to recreate the `VMExternRef`.
+    pub unsafe fn into_raw(self) -> *mut u8 {
+        let ptr = self.0.cast::<u8>().as_ptr();
+        std::mem::forget(self);
+        ptr
+    }
+
     /// Recreate a `VMExternRef` from a pointer returned from a previous call to
-    /// `VMExternRef::as_raw`.
+    /// `as_raw`.
+    ///
+    /// # Safety
+    ///
+    /// Unlike `clone_from_raw`, this does not increment the reference count of the
+    /// underlying data.  It is not safe to continue to use the pointer passed to this
+    /// function.
+    pub unsafe fn from_raw(ptr: *mut u8) -> Self {
+        debug_assert!(!ptr.is_null());
+        VMExternRef(NonNull::new_unchecked(ptr).cast())
+    }
+
+    /// Recreate a `VMExternRef` from a pointer returned from a previous call to
+    /// `as_raw`.
     ///
     /// # Safety
     ///