Add a blanket impl for &mut std::fmt::Write
Add a blanket impl for &mut std::fmt::Write

There is already a corresponding impl for `std::io::Write`. This change
will make the two traits more consistent.

GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust