Add From<u8> for ExitCode

This should cover a mostly cross-platform subset of supported exit codes.

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

@@ -162,6 +162,15 @@ impl ExitCode {
     }
 }
 
+impl From<u8> for ExitCode {
+    fn from(code: u8) -> Self {
+        match code {
+            0 => Self::SUCCESS,
+            1..255 => Self::FAILURE,
+        }
+    }
+}
+
 pub struct Process(!);
 
 impl Process {