feat: Hugrmut on generic nodes

- Allows `HugrMut` to be implemented for `HugrView`s with arbitrary node
  types
- Removes `HugrMutInternals::hugr_mut(&mut self) -> &mut Hugr`, it can
  be implemented for more complex types

drive-by: Fix a couple bugs in rewrite implementations that assumed that
`SiblingMut` contained transitive children.

Author: aborgna-q

hugr-core/src/builder/build_traits.rs

@@ -153,7 +153,7 @@ pub trait Container {
     where
         ExtensionRegistry: Extend<Reg>,
     {
-        self.hugr_mut().extensions_mut().extend(registry);
+        self.hugr_mut().use_extensions(registry);
     }
 }