Memoization code -- not re-entrant!
Returns memoized version of any function with a single argument. The default caching policy is to remember everything for the lifetime of the returned closure, but one may specify an upper bound on cache size. Whenever a cache entry must be forgotten in order to obey this bound, we pick the least-recently-used one.
Raises an exception if cache_size_bound is nonpositive.
efficient special case for argument type unit