Use lookahead token to decide which error message to use

Today when we use menhir's compile_errors option to generate error messages table in ocaml, the only input is the error state. It generates something like:

let message = 
fun s-> 
  match s with
  | 0 -> 
    "syntax error blah blah blah"
  ...

But sometimes for the same state, we want to have different error messages for different lookahead token. I'm thinking maybe we can use lookahead token as another input to error messages. So the generated ocaml error table file would look like this:

let message = 
fun s token -> 
  match (s, token) with
    | (0, MINUSGREATER) ->
      "Doesn't expect '->' here! Maybe you mean '=>' ?" 
    | (0, DIV) -> 
      ...
    | (0, _) ->
     ...

Start by locating the compile_errors implementation that generates the OCaml error-message table; no file or test is named in the issue. Review how the current state-only message function is produced and how lookahead tokens are represented. Done means generated messages can distinguish tokens for the same error state while retaining a fallback for other tokens.