delay exception memoization, deftype cross-protocol method merge, more map-like dispatch

Further clojure.core.cache fixes (198 -> 257 of its assertions):

- delay: a throwing body re-ran on every force and never became realized?. Run it
  once like Clojure's Delay — cache the exception, mark realized, re-throw the same
  on each deref. Fixes value-fn memoization / cache-stampede protection.
- deftype/defrecord: a method name appearing in two protocols with different
  arities (data.priority-map's seq is in IPersistentMap [this] AND Sorted
  [this asc]) registered per-protocol and shadowed; merge clauses by name across
  all protocols into one multi-arity fn.
- empty?/peek/pop (IPersistentStack) dispatch through a deftype's methods; (= a-
  deftype other) uses its equiv method (so caches compare to their backing map);
  seq handles a host iterator (iterator-seq over .iterator).
- pop of an empty PersistentQueue returns it, like the JVM (was an error).

JVM-certified corpus rows. make test + shakesmoke green.

host/chez/natives-queue.ss

@@ -19,7 +19,9 @@
 (define (queue-peek q) (if (null? (jolt-queue-front q)) jolt-nil (car (jolt-queue-front q))))
 (define (queue-pop q)
   (let ((f (jolt-queue-front q)))
-    (cond ((null? f) (error 'pop "can't pop empty queue"))
+    ;; popping an empty PersistentQueue returns it (Clojure's pop: if f==null
+    ;; return this) — unlike a vector, which throws.
+    (cond ((null? f) q)
           ((null? (cdr f)) (make-jolt-queue (reverse (jolt-queue-rear q)) '() (fx- (jolt-queue-cnt q) 1)))
           (else (make-jolt-queue (cdr f) (jolt-queue-rear q) (fx- (jolt-queue-cnt q) 1))))))