`count` is not tail recursive

[LiamPack]

I've run into problems with the count function, which lifts 'a parsers to be 'a list parsers, stack overflowing. This is because the current implementation uses a recursive build-up of lifting cons to parsers in a non-tail-recursive fashion:

let count n p =
  if n < 0 
  then fail "count: n < 0"
  else 
    let rec loop = function
      | 0 -> return []
      | n -> lift2 cons p (loop (n - 1))
    in
    loop n

A naiive fix that I have is the following, which changes the above count to be tail-recursive:

(* A tail-recursive implementation of "count" *)
let count_tr n p =
  if n < 0
  then fail "count: n < 0"
  else
    let rec loop acc =
      function
      | 0 -> acc
      | n -> loop (lift2 cons p acc) (n - 1)
    in
    loop (return []) n