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)
Yeah it's definitely true that you can use `{}` and `{:d}` to differentiate ways to format the same structure today, but from what I've seen it's quite rare. I've also started seeing a number of precedents for functions like `path.display()` where some anonymous struct is returned that only implements one of the formatting traits so you can basically always use `{}` and not have to worry much about it.