Support binary data to stdin of run-external (#4984)

* Add test for passing binary data through externals

This change adds an ignored test to confirm that binary data is passed
correctly between externals to be enabled in a later commit along with
the fix.

To assist in platform agnostic testing of binary data a couple of
additional testbins were added to allow testing on `Value::Binary` inside
`ExternalStream`.

* Support binary data to stdin of run-external

Prior to this change, any pipeline producing binary data (not detected
as string) then feed into an external would be ignored due to
run-external only supporting `Value::String` on stdin.

This change adds binary stdin support for externals allowing something
like this for example:

  〉^cat /dev/urandom | ^head -c 1MiB | ^pv -b | ignore
  1.00MiB

This would previously output `0.00 B [0.00 B/s]` due to the data not
being pushed to stdin at each stage.

src/main.rs

@@ -159,6 +159,8 @@ fn main() -> Result<()> {
                     "echo_env" => test_bins::echo_env(),
                     "cococo" => test_bins::cococo(),
                     "meow" => test_bins::meow(),
+                    "meowb" => test_bins::meowb(),
+                    "relay" => test_bins::relay(),
                     "iecho" => test_bins::iecho(),
                     "fail" => test_bins::fail(),
                     "nonu" => test_bins::nonu(),

src/test_bins.rs

@@ -34,6 +34,25 @@ pub fn meow() {
     }
 }
 
+// A binary version of meow
+pub fn meowb() {
+    let args: Vec<String> = args();
+
+    let stdout = io::stdout();
+    let mut handle = stdout.lock();
+
+    for arg in args.iter().skip(1) {
+        let buf = std::fs::read(arg).expect("Expected a filepath");
+        handle.write_all(&buf).expect("failed to write to stdout");
+    }
+}
+
+// Relays anything received on stdin to stdout
+pub fn relay() {
+    io::copy(&mut io::stdin().lock(), &mut io::stdout().lock())
+        .expect("failed to copy stdin to stdout");
+}
+
 pub fn nonu() {
     args().iter().skip(1).for_each(|arg| print!("{}", arg));
 }