Adequately document that Data.Store.Core is an internal module + avoid IO?

[mgsloan]

As mentioned here, it might be nice to restrict our IO to only memory accesses. I'd only be in favor of doing that if there is no impact to performance.

If not this resolution, we should document that Data.Store.Core is an internal module, and so you can use it to violate invariants (like "doesn't read files") .