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)
Here's an example in the liblog that currently uses the ability to format as a String or a Signed: https://github.com/rust-lang/rust/blob/master/src/liblog/lib.rs#L234-L249

This allows someone to show the log level as the string `INFO` or integer `3`.