[New feature] Syntactic check for let ... in

[david-maison]

I would like to force encourage some developers to use a more readable code style.
One of the most important thing to make the code more readable IMO is about the let .. in construction:
the in keyword should be either on the same line of the let keyword, or alone on its own line.

Example:

(* Valid *)
let a = 0 in
()

(* Valid *)
let a =
  do_something "that need to add a newline to respect the 80chars rule"
in
()

(* Permitted (but I'm not fan about this style) *)
let a = do_something "that need to add a newline to respect the 80chars rule" 
in
()

(* Invalid *)
let a =
  do_something () in
()

(* Invalid *)
let a =
  do_somehtign_with_typos ()
in ()

What do you think about this?