[WIP] Avoid infinite loop when hash-table has no empty members

[Affonso-Gui]

Problem:
Infinite loops when hash-table has no empty members

Code to reproduce:

(defvar ht (make-hash-table))
(dotimes (i (send ht :size))
  (send ht :enter i "foo")
  (send ht :delete i))
(send ht :enter 0 "foo")
;; infinite loop

Why it happens:

• On the first loop the hash-table gets full with del symbols
• The :find method loops until it finds an empty member, which in this case does not exist

Solution:
After all elements are iterated in the :find method, return the first available slot (del symbol). If there are no available slots, raise an error reporting that the hash-table is full.

Thanks to @ikeshou for finding the bug!

[Affonso-Gui]

Debating with @knorth55 about this problem, we realized that another possible solution is to have two different :find methods, one for setting and one for getting.

The getter :find needs to loop through the list until an empty element is found because we have open addressing. However, for the setter :find it suffices to return the first available slot (empty or del) as soon as it is found.

EDIT: we do need to stop looping over the same elements repeatedly anyways

[Affonso-Gui]

Talking to @ikeshou, having to loop once before detecting that the hash is full with deleted elements is not a good approach, and the correct would be to rehash the table based on the non-empty elements (filled + deleted) instead.

Will try to implement that and rebase later (changing to WIP for now)

[Affonso-Gui]

Redirecting to #492