Track call arguments in a single list (#5125)

* Initial implementation of ordered call args

* Run cargo fmt

* Fix some clippy lints

* Add positional len and nth

* Cargo fmt

* Remove more old nth calls

* Good ole rustfmt

* Add named len

Co-authored-by: Hristo Filaretov <h.filaretov@protonmail.com>

crates/nu-command/src/strings/build_string.rs

@@ -54,8 +54,7 @@ impl Command for BuildString {
     ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
         let config = stack.get_config().unwrap_or_default();
         let output = call
-            .positional
-            .iter()
+            .positional_iter()
             .map(|expr| {
                 eval_expression(engine_state, stack, expr).map(|val| val.into_string(", ", &config))
             })

crates/nu-command/src/strings/char_.rs

@@ -245,7 +245,10 @@ impl Command for Char {
             }
             let mut multi_byte = String::new();
             for (i, arg) in args.iter().enumerate() {
-                let span = call.nth(i).expect("Unexpected missing argument").span;
+                let span = call
+                    .positional_nth(i)
+                    .expect("Unexpected missing argument")
+                    .span;
                 multi_byte.push(string_to_unicode_char(arg, &span)?)
             }
             Ok(Value::string(multi_byte, call_span).into_pipeline_data())
@@ -262,7 +265,9 @@ impl Command for Char {
             } else {
                 Err(ShellError::UnsupportedInput(
                     "error finding named character".into(),
-                    call.nth(0).expect("Unexpected missing argument").span,
+                    call.positional_nth(0)
+                        .expect("Unexpected missing argument")
+                        .span,
                 ))
             }
         }