Phase 14 followup: fix 4 core bugs, re-add 5 tests

Bugs fixed:
- core-pr-str: now proper fn with keyword-aware rendering
  (was aliased to core-str which loses the : prefix)
- core-seq nil: return nil instead of empty array
- core-rest nil: return nil instead of @[]
- core-every-pred: new function (stub returning false for now)

Tests re-added:
- section 16: pr-str for keywords (:hello → ":hello")
- section 20: first nil → nil, seq nil → nil
- section 23: var-dynamic? test (function exists since Phase 3)
- section 21: @ deref reader macro (already works)
- section 18: with-meta map equality test

Stubs/deferred:
- every-pred: stub returning false (Phase 15 IFn protocol needed)
- & rest destructuring: Phase 15 (destructuring completion)

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

src/jolt/core.janet

@@ -196,7 +196,7 @@
 (defn core-rest [coll]
   (if (lazy-seq? coll) (ls-rest coll)
     (if (or (nil? coll) (= 0 (length coll)))
-      @[]
+      nil
       (if (tuple? coll)
         (tuple/slice coll 1)
         (array/slice coll 1)))))
@@ -617,6 +617,20 @@
 (def core-print print)
 (def core-println (fn [& xs] (apply print xs) (print "\n") nil))
 
+(defn core-pr-str [& xs]
+  "Returns a string representation of xs suitable for reading."
+  (var buf @"")
+  (var first? true)
+  (each x xs
+    (if first? (set first? false) (buffer/push buf " "))
+    (if (nil? x) (buffer/push buf "nil")
+      (= true x) (buffer/push buf "true")
+      (= false x) (buffer/push buf "false")
+      (keyword? x) (do (buffer/push buf ":") (buffer/push buf (string x)))
+      (string? x) (do (buffer/push buf "\"") (buffer/push buf x) (buffer/push buf "\""))
+      (buffer/push buf (string x))))
+  (string buf))
+
 (defn core-pr [& xs]
   (var i 0)
   (while (< i (length xs))