Suggested default impl for the new *Stacks

In fact they are currently `Default` (can't use it directly due to
`const`)

Maybe an allow is more appriate for stability but seems benign from my
viewpoint

crates/nu-protocol/src/engine/argument.rs

@@ -71,7 +71,7 @@ impl Argument {
 }
 
 /// Stores the argument context for calls in IR evaluation.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct ArgumentStack {
     arguments: Vec<Argument>,
 }

crates/nu-protocol/src/engine/error_handler.rs

@@ -10,7 +10,7 @@ pub struct ErrorHandler {
 }
 
 /// Keeps track of error handlers pushed during evaluation of an IR block.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default)]
 pub struct ErrorHandlerStack {
     handlers: Vec<ErrorHandler>,
 }