Add global memo cache and integrate with the setUp and teardown injection

[maxmynter]

Closes #126
Needs #127

• Integrate Cache into setup and teardown based on state (This is to be done by the user in the setUp and tearDown hooks).

[nicholasjng]

Before you continue: Let's try with a bare dict with lock in nnbench.types.memo as a cache first, since all methods you implemented are class-scoped.

Likewise, getting a memo from cache should prompt a key lookup first:

# this in a decorator applied to Memo.__call__():
_sid = id(self)
if _sid in _memo_cache:
    return _memo_cache[_sid]
return self.__call__()