core: Stage 2 Task 2 tier 5a — compile defmulti + defmethod

defmulti/defmethod become macros (30-macros) over ctx-capturing
clojure.core fns (defmulti-setup/defmethod-setup, interned by
install-stateful-fns!):
- defmulti: (defmulti name dispatch & opts) -> (defmulti-setup 'name
  dispatch ~@opts). name quoted; dispatch + opts (:default/:hierarchy)
  evaluated. defmulti-setup builds the dispatch closure over the method
  table and interns the var (same hierarchy/default/cache behavior).
- defmethod: (defmethod mm dval & fn-tail) -> (defmethod-setup 'mm dval
  (fn ~@fn-tail)). The method impl is now a COMPILED (fn …) (was an
  interpreted fn* eval). Auto-creates the multimethod if missing.
- removed their special-symbol? entries + eval-list arms, and dropped them
  from host_iface special-names + loader stateful-head?.

Both compile + interpret as plain invokes; dispatch incl. :default and
derive/hierarchy works in both modes.

Tests: evaluator-test (defmulti case) + namespace-test now use init (these
forms are overlay macros now, so a bare make-ctx lacks them).

Gate green: conformance 269x3, fallback-zero 38/4, bootstrap-fixpoint
stage1==2==3, self-host, staged-bootstrap, sci-bootstrap, clojure-test-suite
>=4034/67, features 78/78, all unit + spec (multimethods 16/16).

test/unit/evaluator-test.janet

@@ -111,7 +111,9 @@
 (print "  passed")
 
 (print "15: defmulti/defmethod...")
-(let [ctx (make-ctx)]
+# defmulti/defmethod are overlay macros now (Stage 2 jolt-eaa), so this needs the
+# full env (init loads the overlay + installs the *-setup fns), not a bare make-ctx.
+(let [ctx (init)]
   (eval-form ctx @{} (parse-string "(defmulti my-dispatch (fn* [x] (x :type)))"))
   (eval-form ctx @{} (parse-string "(defmethod my-dispatch :foo [_] :got-foo)"))
   (eval-form ctx @{} (parse-string "(defmethod my-dispatch :bar [_] :got-bar)"))