general hashable f returns a memoized version of f, where the results are stored
in a hash table indexed according to hashable. If f a asynchronously raises, then
the error is stored in the hash table and is reraised when a is demanded.
Unlike Core_kernel.Std.Memo.general, this 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
determine it.
Unlike Core_kernel.Std.Memo.general, this general takes a required Hashable
module argument, to avoid unintentional use of polymorphic comparison.