memory.c: hashtables
native.c hashtables interface
ht-fold: hashtable iterator

Usage:

```Scheme
    (define ht (ht-make))
    (for-each
      (lambda (kv) (ht-set! ht (car kv) (cdr kv)))
      '((one . 1) (two . 2) (three . 3) (four . 4) (five . 5)))

    (ht-fold
       ht           ;; hashtable to fold
       #t           ;; initial state
       (lambda (kv state) (display kv) (newline) #t))
```