Implement Sealed for ExitStatus only once.

With cfg(doc), both the windows and unix implementations were included,
which conflicts. This moves the implementation to the file that defines
the type.

Author: m-ou-se

library/std/src/process.rs

@@ -1375,6 +1375,10 @@ impl From<fs::File> for Stdio {
 #[stable(feature = "process", since = "1.0.0")]
 pub struct ExitStatus(imp::ExitStatus);
 
+/// Allows extension traits within `std`.
+#[unstable(feature = "sealed", issue = "none")]
+impl crate::sealed::Sealed for ExitStatus {}
+
 impl ExitStatus {
     /// Was termination successful? Signal termination is not considered a
     /// success, and success is defined as a zero exit status.

library/std/src/sys/unix/ext/process.rs

@@ -203,9 +203,6 @@ pub trait ExitStatusExt: Sealed {
     fn into_raw(self) -> i32;
 }
 
-#[unstable(feature = "sealed", issue = "none")]
-impl Sealed for process::ExitStatus {}
-
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ExitStatusExt for process::ExitStatus {
     fn from_raw(raw: i32) -> Self {

library/std/src/sys/windows/ext/process.rs

@@ -92,9 +92,6 @@ impl ExitStatusExt for process::ExitStatus {
     }
 }
 
-#[unstable(feature = "sealed", issue = "none")]
-impl Sealed for process::ExitStatus {}
-
 /// Windows-specific extensions to the [`process::Command`] builder.
 #[stable(feature = "windows_process_extensions", since = "1.16.0")]
 pub trait CommandExt {