ListStream touchup (#12524)

# Description

Does some misc changes to `ListStream`:
- Moves it into its own module/file separate from `RawStream`.
- `ListStream`s now have an associated `Span`.
- This required changes to `ListStreamInfo` in `nu-plugin`. Note sure if
this is a breaking change for the plugin protocol.
- Hides the internals of `ListStream` but also adds a few more methods.
- This includes two functions to more easily alter a stream (these take
a `ListStream` and return a `ListStream` instead of having to go through
the whole `into_pipeline_data(..)` route).
  -  `map`: takes a `FnMut(Value) -> Value`
  - `modify`: takes a function to modify the inner stream.

crates/nu_plugin_example/src/commands/generate.rs

@@ -60,8 +60,8 @@ impl PluginCommand for Generate {
         call: &EvaluatedCall,
         _input: PipelineData,
     ) -> Result<PipelineData, LabeledError> {
+        let head = call.head;
         let engine = engine.clone();
-        let call = call.clone();
         let initial: Value = call.req(0)?;
         let closure = call.req(1)?;
 
@@ -83,9 +83,9 @@ impl PluginCommand for Generate {
                         })
                         .transpose()
                 })
-                .map(|result| result.unwrap_or_else(|err| Value::error(err, call.head)))
+                .map(|result| result.unwrap_or_else(|err| Value::error(err, head)))
         })
-        .into_pipeline_data(None))
+        .into_pipeline_data(head, None))
     }
 }
 

crates/nu_plugin_example/src/commands/seq.rs

@@ -50,12 +50,11 @@ impl PluginCommand for Seq {
         call: &EvaluatedCall,
         _input: PipelineData,
     ) -> Result<PipelineData, LabeledError> {
+        let head = call.head;
         let first: i64 = call.req(0)?;
         let last: i64 = call.req(1)?;
-        let span = call.head;
-        let iter = (first..=last).map(move |number| Value::int(number, span));
-        let list_stream = ListStream::from_stream(iter, None);
-        Ok(PipelineData::ListStream(list_stream, None))
+        let iter = (first..=last).map(move |number| Value::int(number, head));
+        Ok(ListStream::new(iter, head, None).into())
     }
 }