Add `impl Show for &Show`

This makes printing a `Show` trait object much easier.

src/libcore/fmt/mod.rs

@@ -518,6 +518,9 @@ impl<'a, T: Show> Show for &'a T {
 impl<'a, T: Show> Show for &'a mut T {
     fn fmt(&self, f: &mut Formatter) -> Result { secret_show(&**self, f) }
 }
+impl<'a> Show for &'a Show {
+    fn fmt(&self, f: &mut Formatter) -> Result { (*self).fmt(f) }
+}
 
 impl Bool for bool {
     fn fmt(&self, f: &mut Formatter) -> Result {

src/test/run-pass/ifmt.rs

@@ -81,6 +81,9 @@ pub fn main() {
     t!(format!("{foo_bar}", foo_bar=1i), "1");
     t!(format!("{:d}", 5i + 5i), "10");
 
+    let a: &fmt::Show = &1i;
+    t!(format!("{}", a), "1");
+
     // Formatting strings and their arguments
     t!(format!("{:s}", "a"), "a");
     t!(format!("{:4s}", "a"), "a   ");