Name threads (#7879)

I noticed that [it's pretty easy to name threads in
Rust](https://doc.rust-lang.org/std/thread/#naming-threads). We might as
well do this; it's a nice quality of life improvement when you're
profiling something and the developers took the time to give threads
names.

Also added/cleaned up some comments while I was in the area.

crates/nu-command/src/filesystem/save.rs

@@ -8,6 +8,7 @@ use nu_protocol::{
 use std::fs::File;
 use std::io::{BufWriter, Write};
 use std::path::Path;
+use std::thread;
 
 use crate::progress_bar;
 
@@ -85,13 +86,17 @@ impl Command for Save {
 
                 // delegate a thread to redirect stderr to result.
                 let handler = stderr.map(|stderr_stream| match stderr_file {
-                    Some(stderr_file) => std::thread::spawn(move || {
-                        stream_to_file(stderr_stream, stderr_file, span, progress)
-                    }),
-                    None => std::thread::spawn(move || {
-                        let _ = stderr_stream.into_bytes();
-                        Ok(PipelineData::empty())
-                    }),
+                    Some(stderr_file) => thread::Builder::new()
+                        .name("stderr redirector".to_string())
+                        .spawn(move || stream_to_file(stderr_stream, stderr_file, span, progress))
+                        .expect("Failed to create thread"),
+                    None => thread::Builder::new()
+                        .name("stderr redirector".to_string())
+                        .spawn(move || {
+                            let _ = stderr_stream.into_bytes();
+                            Ok(PipelineData::empty())
+                        })
+                        .expect("Failed to create thread"),
                 });
 
                 let res = stream_to_file(stream, file, span, progress);