[REFACTOR][RUST] Keep mutate dispatch state on the dispatcher

[tlopex]

This PR makes typed structural mutation dispatch own its pass state directly.

Previously, #[dispatch(mutate)] separated the dispatch object from a State stored inside MutateCallbacks, requiring handlers to access state through mutator.state() or mutator.state_mut().

Now the state lives naturally on the dispatch object:

struct MyMutator {
    analyzer: Analyzer,
    // Other pass state.
}

#[tvm_ffi::dispatch(mutate)]
impl MyMutator {
    fn mutate_node(
        &mut self,
        value: Node,
        mutator: &mut Mutator,
    ) -> Result<Node> {
        let child = mutator.mutate(self, &value.child)?;
        // Rewrite or reuse the node.
    }
}

The responsibilities are now separated as follows:

• The dispatch object owns pass-specific state.
• #[dispatch(mutate)] selects the matching typed handler.
• Mutator provides recursion, default mutation, variable remapping, and the current definition region.
• Explicitly passing self during recursion creates a normal Rust mutable reborrow and avoids hidden aliasing through raw pointers.
• Stateful closure callback chains continue to be supported through CallbackMutator<State> and MutateCallbacks.

The Rust guide and structural-mutation tests have been updated to use the new API. All relevant Rust tests, including the 47 structural-mutation tests, pass.