Add a test that bare functions are word-sized

Issue rust-lang#1022

Author: brson

src/test/run-pass/fn-bare-size.rs

@@ -0,0 +1,6 @@
+use std;
+
+fn main() {
+    assert std::sys::rustrt::size_of::<fn#()>() ==
+        std::sys::rustrt::size_of::<int>();
+}