A ppx rewriter that inlines reverse application operators |> and |!.

ppx_pipebang rewrites x |> f and x |! f as f x, regardless of whether |> and
|! have been redefined.

Do not use |! in new code, it is there only for backwards compatibility.

This inlining is mostly done for historical reasons but it also allows f to have
optional arguments (like Option.value_exn).