Use ZST for fmt unsafety
Use ZST for fmt unsafety

This allows the format_args! macro to keep the pre-expansion code out of
the unsafe block without doing gymnastics with nested `match`
expressions. This reduces codegen.

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