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.
auto merge of #11948 : huonw/rust/show, r=alexcrichton

- renames `Default` to `Show`
- introduces some hidden `std::fmt::secret_...` functions, designed to work-around the lack of UFCS (with UFCS they can be replaced by referencing the trait methods directly) because I'm going to convert the traits to have methods rather than static functions, since `#[deriving]` works much better with true methods.

I'm blocked on a snapshot after this. (I could probably do a large number of `#[cfg]`s, but I can work on other things in the meantime.)