core.async: higher-level API over native channels + two general fixes

Adds clojure.core.async's higher-level dataflow API as a Clojure overlay
(stdlib/clojure/core/async.clj) over jolt's native channel primitives, plus
clojure.core.async.lab. The native layer (host/chez/java/async.ss) gains
offer!/poll!, put specs and :priority/:default in alts!, a transducer
ex-handler arg to chan, unblocking-buffer?, promise-buffer, and on-caller?
handling for put!/take!. The overlay covers alts!/pipe/pipeline/split/
reduce/transduce/into/take/mult/mix/pub-sub/map/merge/onto-chan/to-chan and
the deprecated map</map>/filter>/... family (rewritten as go-loops since the
JVM versions reify the impl handler protocol jolt doesn't expose).

Loading: the native primitives pre-seed clojure.core.async, so the loader now
drops it from the loaded set and a require pulls the overlay from the source
roots like clojure.test (AOT-bundled into built binaries).

Running clojure/core.async's own suite shook out two general bugs:
- :refer with a list form, (:require [ns :refer (a b c)]), dropped the names
  (only the vector form was handled) — chez-register-spec! now accepts both.
- (range 0) / (range 5 5) returned nil instead of the empty seq () — empty
  ranges now match Clojure, so (= () (range 0)) holds.

Suite: async_test 15/20, pipeline_test 7/7, timers_test 2/2, lab_test 2/2.
The five non-passing async_test cases all assert JVM go-machine limitations
jolt's thread-based model is a superset of (the 1024 pending-op cap, parking
ops that must throw outside a go block, expanding-transducer buffer
backpressure) or dispatch-thread identity, not data semantics.

make test green (0 new divergences, +4 range corpus rows), shakesmoke
byte-identical.

host/chez/loader.ss

@@ -141,6 +141,14 @@
 (vector-for-each (lambda (c) (hashtable-set! loaded-ns (var-cell-ns c) #t))
                  (hashtable-values var-table))
 
+;; clojure.core.async ships native channel primitives (async.ss) AND a Clojure
+;; overlay (stdlib/clojure/core/async.clj) with the higher-level dataflow API
+;; (alts!, pipe, mult, mix, pub/sub, map, merge, …). The primitives pre-seed the
+;; namespace above, which would make a `require` no-op and skip the overlay. Drop
+;; it from the loaded set so a require pulls the overlay from the source roots
+;; (like clojure.test); the primitives stay defined either way.
+(hashtable-delete! loaded-ns "clojure.core.async")
+
 ;; Does `name` already have vars in the var-table? A namespace baked into the
 ;; image after the snapshot above — an AOT'd app namespace in a `jolt build`
 ;; binary — exists in memory with no source file; a later `require` of it must