general hashable f returns a memoized version of
f, where the results are stored
in a hash table indexed according to
f a asynchronously raises, then
the error is stored in the hash table and is reraised when
a is demanded.
general does not support
cache_size_bound due to complexities of asynchrony -- even when one has a deferred
return by the memoized function, there still may be asynchronous jobs working to
general takes a required
module argument, to avoid unintentional use of polymorphic comparison.