Add one-line comment support and make some semi-column optional

Make possible the parsing of the counting example of *Clock-directed Modular Code Generation for Synchronous Data-flow Languages* (https://www.di.ens.fr/~pouzet/bib/lctes08a.pdf).

```
-- count the number of top between two tick
node counting (tick:bool; top:bool)
returns (o: int)
var v: int;
let o = if tick then v else 0 -> pre o + v;
    v = if top then 1 else 0;
tel;
```

The one-line comment rule was inspired from https://mukulrathi.com/create-your-own-programming-language/parsing-ocamllex-menhir/. Note their typo using `single_line_comment` instead of `read_single_line_comment`.

src/lexer.mll

@@ -62,6 +62,11 @@ rule token = parse
   | '/'             { BO_div }
   | '%'             { BO_mod }
   | "->"            { BO_arrow }
+  | "--"            { read_single_line_comment lexbuf }
   | eof             { EOF }
   | _               { raise (Lexing_error (Format.sprintf "Error when seeing %s" (lexeme lexbuf)))}
 
+and read_single_line_comment = parse
+  | '\n' { token lexbuf }
+  | eof { EOF }
+  | _ { read_single_line_comment lexbuf }