This cache is more general than the previous cache and also better
abstracted in that it allows storing anything into it, and permits
configuration of it from outside the contract runtime.

Also no global statics (other than for the empty cache sentinel.)

Simplifies the code quite a bit.

The blast radius... Thank dog I decided against upgrading lru for the
entire workspace or I'd be sitting doing just that till deep winter.

Alas this didn't pan out quite yet -- instantiation still requires an
`Arc<Artifact>` and removing that requirement is not the right time
right now