with the help of two classes. The MemoCache is not intended to be
instantiated but holds a global cache. One can interact with this
cache via classmethods.

The `Memo` class is to be used as a decorator on whatever one wants
to cache. It computes a value lazily and caches it within
`MemoCache`.

You can delete the class to empty the cache.

Also some very rough memo cache manip APIs.

This means all required parts are in place for user memory management
with memos.