Remove uses of assert-c! and simplify docstrings now that conditional expansion works properly

Previously, we had to separate platform-dependent code that was called during macroexpansion from code called on the client, since the old `compiling-cljs` couldn't tell the difference between Clojure compilation and ClojureScript generation.  The new `if-cljs` can tell the difference, so error handling can be simplified.
Maybe use ^:deprecated metadata? (Logs deprecation warnings)