compiler: direct-linking (call-site/unit, Clojure model) + :aot-core? flag

Calls compile direct (target value embedded) iff the compiling unit has
direct-linking on, the target isn't ^:redef/^:dynamic, and it's an
already-defined fn; otherwise indirect (live var deref, redefinable). Direct
emission is guarded to function values only — embedding a non-function would make
Janet evaluate it as code.

:aot-core? (init opt, default true; JOLT_AOT_CORE=0 disables) compiles the core
tiers with direct-linking on so core->core calls are direct. User/REPL code
compiles indirect by default, so redefining functions in the REPL works with no
annotation, exactly like Clojure. :aot-core? false makes core indirect too — the
whole language becomes redefinable. ^:redef / ^:dynamic on a target force it
indirect even inside a direct-linked unit.

Also fixes a pre-existing gap this surfaced: compiled def dropped ALL var
metadata (so ^:dynamic was silently lost under compilation, and ^:redef couldn't
work). def metadata now flows analyzer -> IR -> back end (new form-sym-meta host
fn, def-node :meta, var-setter-meta) and is applied to the cell.

Matrix test in test/integration/direct-linking-test.janet. Conformance 218/218
in all three modes under both :aot-core? true and false; battery holds 3916;
binary rebuilt.

jolt-core/jolt/analyzer.clj

@@ -22,7 +22,7 @@
                                form-literal? form-elements form-vec-items
                                form-map-pairs form-special? compile-ns
                                form-macro? form-expand-1 resolve-global
-                               host-intern!]]))
+                               form-sym-meta host-intern!]]))
 
 (declare analyze)
 
@@ -140,7 +140,7 @@
                 nm (form-sym-name name-sym)
                 cur (compile-ns ctx)]
             (host-intern! ctx cur nm)
-            (def-node cur nm (analyze ctx (nth items 2) env)))
+            (def-node cur nm (analyze ctx (nth items 2) env) (form-sym-meta name-sym)))
     "let*" (let [bvec (vec (form-vec-items (nth items 1)))
                  r (analyze-bindings ctx bvec env)]
              (let-node (first r) (analyze-seq ctx (drop 2 items) (second r))))

jolt-core/jolt/ir.clj

@@ -29,7 +29,11 @@
 
 (defn invoke [f args] {:op :invoke :fn f :args args})
 
-(defn def-node [ns name init] {:op :def :ns ns :name name :init init})
+;; meta is the var metadata (e.g. {:dynamic true} / {:redef true}) the back end
+;; applies to the cell; nil when the def name carried none.
+(defn def-node
+  ([ns name init] {:op :def :ns ns :name name :init init :meta nil})
+  ([ns name init meta] {:op :def :ns ns :name name :init init :meta meta}))
 
 (defn let-node [bindings body] {:op :let :bindings bindings :body body})