Convert a value to a human-readable String — what a person should read.

This is the counterpart of show: show is faithful and round-trippable (it looks like valid oppi code — show "hi" = "\"hi\"", with the quotes), whereas display is the human form (display "hi" = "hi", raw). For most types the two agree, so every Show type is a Display for free through the blanket below; a type gives its own display only when the human form really differs (a String is the first such case).