RFC: Stabilize `std::fmt`

High-level summary:

* Leave the format syntax as-is.
* Remove a number of superfluous formatting traits (renaming a few in the process).


[Rendered](https://github.com/alexcrichton/rfcs/blob/stdfmt/active/0000-stabilize-std-fmt.md)
`{:u}` and other formatters have an important function of parameter typechecking. E. g. code like this:

```
println!("Found {:u} files", count);
```

didn't compile after I changed `count` from `uint` to `Option<uint>` during refactoring. That's what I expect from statically typed language.

And now I have to write:

```
println!("Found {} files", count);
```

and program happily prints something like `Found Some(36) files`.