add forwarding specializations for &mut variants
add forwarding specializations for &mut variants

`impl Write for &mut T where T: Write`, thus the same should
apply to the specialization traits

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