refactor: aztec-macros return values

[LHerskind]

When the macros for return values are applied they will current "copy" the node into the return_values of the context.

This means that for functions like:

#[aztec(private)]
fn is_valid(message_hash: Field) -> Field {
    let actions = AccountActions::private(&mut context, ACCOUNT_ACTIONS_STORAGE_SLOT, is_valid_impl);
    actions.is_valid(message_hash)
}

We will really be executing something along the lines of:

#[aztec(private)]
fn is_valid(message_hash: Field) -> Field {
    let actions = AccountActions::private(&mut context, ACCOUNT_ACTIONS_STORAGE_SLOT, is_valid_impl);
    actions.is_valid(message_hash);
    context.return_values.push(actions.is_valid(message_hash));
}

This executes the function is_valid twice. Which can be "fatal" in some cases where the function is emitting nullifiers as it would create an instant double-spend, (two nullifiers in the same tx).

---

Point of interest:

aztec-packages/noir/aztec_macros/src/lib.rs

Lines 637 to 640 in 811d767

	// Abstract return types such that they get added to the kernel's return_values
	if let Some(return_values) = abstract_return_values(func) {
	func.def.body.0.push(return_values);
	}