Add the Fd definition file.

Damn, I forgot to `git add` it.

Add the Fd definition file.
4966842d · ticki · 9ca3559c · 4966842d
Commit 4966842d authored 8 years ago by ticki
--- a/scheme/fd.rs
+++ b/scheme/fd.rs
+/// A file descriptor.
+#[derive(PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Fd {
+    inner: usize,
+}