Improve the discussion of testing dependencies for staleness.
Improvements to clarity of weak hash table implementation:
* TAG_SIZE was changed to TAG_COUNT in the code, so make the corresponding change in the manual.
* Avoid the "+= 2" hack: use TAG_COUNT(UNTAG_COUNT(...) + 1). Clarity is more important than speed here.
* Use obj_unused for unused hash table entries so that there can be no confusion between NULL meaning "unused" and NULL meaning "splatted by the garbage collector".
* Add references from the example code to the notes.
Fix typo.