Heuristic for deciding when to use fast-alists.
(worth-hashing x) → bool
When alists are very short, it may be better for performance and memory usage to use naive alist algorithms instead of constructing hash tables.
(worth-hashing x) is a rough heuristic that is used in various fast-alist operations (e.g., fal-all-boundp) to decide when alists are long enough or will be used heavily enough to justify constructing hash tables.
It currently just decides whether