Enforce `StoreKey`'s `.borrow()` shallow cloned-ness in the type system

[caass]

I ran into a hiccup where StoreKey is actually a smart pointer to some data on the heap, and I was pasing around references to it -- essentially, references to a reference. If we really want to make StoreKey be a smart pointer, we should document it and talk about how .clone() is cheap or something. Or like, maybe we just impl deref on it or something. Basically, I'm upset that StoreKey doesn't seem like it'd be cheaply cloneable.