CLJS-793: fix memoize (non-truthy values don't get cached)

ClojureScript's memoize fn currently uses `(get @mem args)` to
check for the existence of a cache entry, preventing falsey
values from being cached correctly.

Clojure's `memoize` uses `find` rather than `get` to avoid this
issue. Here we'll use a `get` lookup sentinel since it's faster.