Support for caching the output of rule right-hand-side (RHS) activations
so it can be replayed across rules sessions instead of recomputed.

The engine consults a cache the caller supplies via the `:activation-cache`
fire-rules option: an atom wrapping any `clojure.core.cache/CacheProtocol` implementation.
A rule opts in via its own `:cache` prop. This namespace provides
`build-cache-key`, which derives the key an activation is cached under (callers
can override it per fire with the `:activation-cache-key-fn` fire-rules option). Any
policy beyond basic lookup/hit/miss — TTL, tracking which entries were
consulted, sweeping at the end of a run — is the responsibility of the caller's
cache implementation.