Create unified Traits that can be used for any formatting library

Hello, I am the author of the strfmt library.

I would like to push the stdlib to implement `fmt::Formatter` as a trait instead of a struct so that libraries like ours can use (and build off of) a common API, as well as format any object which implements the `fmt::Display` trait.

This issue is to bring this to your attention and try to work together on it.

vitiral/strfmt#11