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
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.
Remove the SNAP line to work around #11985.
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.)
std: Remove io::io_error

* All I/O now returns IoResult<T> = Result<T, IoError>
* All formatting traits now return fmt::Result = IoResult<()>
* The if_ok!() macro was added to libstd
extra: Remove io_error usage
syntax: Remove io_error usage
rustc: Remove io_error usage
compiletest: Remove io_error usage
glob: Remove io_error usage
green: Remove io_error usage
native: Remove io_error usage
rustuv: Remove io_error usage
rustpkg: Remove io_error usage
rustdoc: Remove io_error usage
rpass: Remove io_error usage
std: Fix tests with io_error usage
extra: Fix tests with io_error usage
native: Require all results are used and fix fallout
rustuv: Require all results are used and fix fallout