std::process (unsupported): Implement From<io::Stdout> etc. for imp::Stdio

This implementation is wrong.  Like the impl for From<File>, it is
forced to panic because process::Stdio in unsupported/process.rs
doesn't have a suitable variant.

The root cause of the problem is that process::Stdio in
unsupported/process.rs has any information in it at all.

I'm pretty sure that it should just be a unit struct.  However,
making that build on all platforms is going to be a lot of work,
iterating through CI and/or wrestling Docker.

I don't think this extra panic is making things significantly worse.
For now I have added some TODOs.

library/std/src/sys/unsupported/process.rs

@@ -27,6 +27,8 @@ pub struct StdioPipes {
     pub stderr: Option<AnonPipe>,
 }
 
+// FIXME: This should be a unit struct, so we can always construct it
+// The value here should be never used, since we cannot spawn processes.
 pub enum Stdio {
     Inherit,
     Null,
@@ -87,8 +89,26 @@ impl From<AnonPipe> for Stdio {
     }
 }
 
+impl From<io::Stdout> for Stdio {
+    fn from(_: io::Stdout) -> Stdio {
+        // FIXME: This is wrong.
+        // Instead, the Stdio we have here should be a unit struct.
+        panic!("unsupported")
+    }
+}
+
+impl From<io::Stderr> for Stdio {
+    fn from(_: io::Stderr) -> Stdio {
+        // FIXME: This is wrong.
+        // Instead, the Stdio we have here should be a unit struct.
+        panic!("unsupported")
+    }
+}
+
 impl From<File> for Stdio {
     fn from(_file: File) -> Stdio {
+        // FIXME: This is wrong.
+        // Instead, the Stdio we have here should be a unit struct.
         panic!("unsupported")
     }
 }