Correct the clippy::use_self lint where possible. (#114)

src/sys/unix/fdentry_impl.rs

@@ -21,10 +21,10 @@ cfg_if::cfg_if! {
 impl AsRawFd for Descriptor {
     fn as_raw_fd(&self) -> RawFd {
         match self {
-            Descriptor::OsFile(file) => file.as_raw_fd(),
-            Descriptor::Stdin => io::stdin().as_raw_fd(),
-            Descriptor::Stdout => io::stdout().as_raw_fd(),
-            Descriptor::Stderr => io::stderr().as_raw_fd(),
+            Self::OsFile(file) => file.as_raw_fd(),
+            Self::Stdin => io::stdin().as_raw_fd(),
+            Self::Stdout => io::stdout().as_raw_fd(),
+            Self::Stderr => io::stderr().as_raw_fd(),
         }
     }
 }