Quoted extensions are lighter syntax for delimited strings in extension points: {%foo|...|} is lighter syntax for [%foo {||}], and {%foo bar|...|bar} for [%foo {bar|...|bar}].

In order not to output code that's invalid on 4.10, I suggest that we output the sugar version only when it was already present in the input source.

Relevant ocaml PRs: ocaml/ocaml#8820, ocaml/ocaml#9166.