defn docstrings, assert throws AssertionError, Seqable covers collections

Conformance gaps surfaced re-running the library suites:

- defn now keeps a leading docstring as :doc metadata — it was dropped, so
  (:doc (meta #'f)) was always nil. Rides the def docstring slot.
- assert (and :pre/:post) throw a real AssertionError instead of an ex-info, so
  (catch AssertionError …) / (thrown? AssertionError …) match, with Clojure's
  "Assert failed: <msg>\n<form>" message.
- instance? clojure.lang.Seqable was conflated with ISeq, so a vector/map read
  as not-Seqable. Split them: Seqable covers every persistent collection, ISeq
  only seqs.

jolt-core/clojure/core/30-macros.clj

@@ -208,8 +208,10 @@
     `(let [~g ~expr ~@(thread-binds g steps)] ~(if (empty? steps) g (last steps)))))
 
 (defmacro assert [x & [message]]
-  (let [msg (if message message (str "Assert failed: " (pr-str x)))]
-    `(when-not ~x (throw (ex-info ~msg {})))))
+  (let [msg (if message
+              (str "Assert failed: " message "\n" (pr-str x))
+              (str "Assert failed: " (pr-str x)))]
+    `(when-not ~x (throw (new AssertionError ~msg)))))
 
 ;; (pvalues e1 e2 ...) — each expression evaluated in parallel (pcalls).
 (defmacro pvalues [& exprs]