(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.
nil (declare (xargs :guard t))