This commit is BREAKING for those still actively using `nippy/cache`.
Data previously frozen using `nippy/cache` can still be thawed, though
support for thawing may also be removed in a future Nippy version.
Motivation for removal:
This cache feature (marked as experimental) was always a bit dubious.
The use cases were very limited, and the complexity quite significant.
I don't believe that the feature has ever had much (any?) public
adoption, so I'm removing it here.
PLEASE LET ME KNOW if this removal negatively impacts you.
648d5c0232