Phase 14: add core-pr-str function + pr-str binding

- core.janet: add core-pr-str (produces Clojure-readable strings:
  nil→nil, strings→"...", keywords→:kw, booleans→true/false,
  numbers→string)
- core.janet: add pr-str to core-bindings
- compiler.janet: fix core-fn-values mapping (core-pr-str now
  resolves to actual core-pr-str function, not core-str)

316 ok, 1 fail (pre-existing, unchanged)

src/jolt/core.janet

@@ -1276,6 +1276,7 @@
     "println" core-println
     "pr" core-pr
     "prn" core-prn
+    "pr-str" core-pr-str
     # Array primitives (for persistent data structures)
     "alength" core-alength
     "aget" core-aget