Implement Add on Option types

This will allow you to use the + operator to add together any two
Options, assuming that the contents of each Option likewise implement
+. So Some(4) + Some(1) == Some(5), and adding with None leaves the
other value unchanged.

This might be monoidic? I don't know what that word means!

src/test/run-pass/option_addition.rs

@@ -0,0 +1,27 @@
+fn main() {
+    let foo = 1;
+    let bar = 2;
+    let foobar = foo + bar;
+
+    let nope = optint(0) + optint(0);
+    let somefoo = optint(foo) + optint(0);
+    let somebar = optint(bar) + optint(0);
+    let somefoobar = optint(foo) + optint(bar);
+
+    match nope {
+        None => (),
+        Some(foo) => fail!(fmt!("expected None, but found %?", foo))
+    }
+    fail_unless!(foo == somefoo.get());
+    fail_unless!(bar == somebar.get());
+    fail_unless!(foobar == somefoobar.get());
+}
+
+fn optint(in: int) -> Option<int> {
+    if in == 0 {
+        return None;
+    }
+    else {
+        return Some(in);
+    }
+}