Use threads to avoid blocking reads/writes in externals. (#1440)

In particular, one thing that we can't (properly) do before this commit
is consuming an infinite input stream. For example:

```
yes | grep y | head -n10
```

will give 10 "y"s in most shells, but blocks indefinitely in nu. This PR
resolves that by doing blocking I/O in threads, and reducing the `await`
calls we currently have in our pipeline code.

crates/nu-test-support/src/bins/chop.rs

@@ -1,4 +1,4 @@
-use std::io::{self, BufRead};
+use std::io::{self, BufRead, Write};
 
 fn main() {
     if did_chop_arguments() {
@@ -8,9 +8,12 @@ fn main() {
 
     // if no arguments given, chop from standard input and exit.
     let stdin = io::stdin();
+    let mut stdout = io::stdout();
     for line in stdin.lock().lines() {
         if let Ok(given) = line {
-            println!("{}", chop(&given));
+            if let Err(_e) = writeln!(stdout, "{}", chop(&given)) {
+                break;
+            }
         }
     }
 

crates/nu-test-support/src/bins/iecho.rs

@@ -0,0 +1,13 @@
+use std::io::{self, Write};
+
+fn main() {
+    let args: Vec<String> = std::env::args().collect();
+
+    // println! panics if stdout gets closed, whereas writeln gives us an error
+    let mut stdout = io::stdout();
+    let _ = args
+        .iter()
+        .skip(1)
+        .cycle()
+        .try_for_each(|v| writeln!(stdout, "{}", v));
+}