std: rename fmt::Default to Show.

This is a better name with which to have a #[deriving] mode.

Decision in:
https://github.com/mozilla/rust/wiki/Meeting-weekly-2014-01-28

src/librustdoc/html/layout.rs

@@ -24,7 +24,7 @@ pub struct Page<'a> {
     root_path: &'a str,
 }
 
-pub fn render<T: fmt::Default, S: fmt::Default>(
+pub fn render<T: fmt::Show, S: fmt::Show>(
     dst: &mut io::Writer, layout: &Layout, page: &Page, sidebar: &S, t: &T)
 {
     write!(dst,