std: rename fmt::Default to `Show`.

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

This was decided in:
https://github.com/mozilla/rust/wiki/Meeting-weekly-2014-01-28
std::fmt: prepare to convert the formatting traits to methods, and work
around the lack of UFCS.

The further work is pending a snapshot, to avoid putting #[cfg(stage0)]
attributes on all the traits and duplicating them.