Inconsistent use of the Index argument in treelist

[yiyunliu]

Currently, both treelist-ref and treelist-set take an index of type Index instead of Integer, which is not only inconsistent with how other collections handle indices (e.g. string-ref, vector-ref, and vector-set! all take an Integer as their argument to index into the container), but also makes it difficult to do arithmetic on indices.

#lang typed/racket
(require racket/treelist)

(: idx Index)
(define idx 0)

(: tr (TreeListof String))
(define tr (treelist "apple" "orange"))

;; okay
(treelist-ref tr idx)
;; not okay, (add1 idx) is no longer an Index
(treelist-ref tr (add1 idx))
;; okay, but annoying
(let ([new-idx (add1 idx)])
  (if (index? new-idx)
      (treelist-ref tr new-idx)
      (error "not an index")))

The same issue doesn't apply if those functions took Integer as inputs instead.