Support let-punning #1
Description
let-punning was introduced in OCaml 4.13.0 as a shorthand notation for variable bindings:
let+ variable = variable in ... can be reduced to let+ variable in
(see https://ocaml.org/manual/bindingops.html#ss:letops-punning)
It would be useful to support that in the future_syntax preprocessor of dune.