(fast-alist-summary) prints some basic statistics about any
current fast alists.   Logically, fast-alist-summary just
returns nil; we leave it enabled and would think it odd to ever
prove a theorem about it.  Under the hood, this function gathers
and prints some basic statistics about the current fast alists.
It may be useful for identifying fast alists that should have
been freed with fast-alist-free.