Exporting some version of Cache

[ratmice]

In PR #41 it was mentioned

We should probably consider eventually exposing some version of the Cache implemented here, such that bottom-up builders can use it and get the deduplication for free.

This would be seem very helpful in trying to integrate lalrpop.

[ratmice]

FWIW i've set up a temporary test repository with a lalrpop variation of the calc example,
https://github.com/ratmice/crispy-parakeet

It has branches for master, PR #60, and sorbus

I still need to look at making a branch based on PR #63, and the cache mechanism already in sorbus, will try and do that soon.