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)
OK, it is not typechecking, let's call it parameter validation.

Although format modifier `{:u}` can be implemented for any type, in practice, it won't be.

I'm repeating myself, but have to tell one more argument. The important difference between printing numbers and strings and printing anything that implements `Show` is that developers print numbers and strings much more often than print all other types. So having special cases `{:u}` and `{:s}` seems reasonable to me.