fix: catch binds unwrapped thrown value; var-set targets thread binding

Close jolt-dd5.
- catch now binds the originally-thrown value (unwrapping the :jolt/exception
  envelope), so (catch ... e (throw e)) rethrows the same exception instead of
  nesting another envelope, and (catch ... e e) on (throw 42) yields 42.
- var-set updates the innermost thread-binding frame for the var (replacing the
  stack slot) when the var is dynamically bound, matching Clojure; it falls back
  to the root otherwise.

Restored spec cases: exceptions rethrow + catch-binds-thrown-value, namespaces
var-set-in-binding. conformance 218/218, jpm test green.

test/spec/exceptions-spec.janet

@@ -25,4 +25,8 @@
   ["ex-cause"            "true"
    "(let [c (ex-info \"root\" {})] (= c (ex-cause (ex-info \"outer\" {} c))))"]
   ["propagates to outer"  "\"inner\""
-   "(try (try (throw (ex-info \"inner\" {})) (finally nil)) (catch :default e (ex-message e)))"])
+   "(try (try (throw (ex-info \"inner\" {})) (finally nil)) (catch :default e (ex-message e)))"]
+  ["catch binds thrown value" "42"
+   "(try (throw 42) (catch :default e e))"]
+  ["rethrow preserves ex"  "\"inner\""
+   "(try (try (throw (ex-info \"inner\" {})) (catch :default e (throw e))) (catch :default e (ex-message e)))"])

test/spec/namespaces-spec.janet

@@ -11,7 +11,8 @@
   ["defn defines fn"    "3"        "(do (defn f [x] (inc x)) (f 2))"]
   ["def with docstring" "7"        "(do (def d \"a doc\" 7) d)"]
   ["dynamic var binding" "2"       "(do (def ^:dynamic *x* 1) (binding [*x* 2] *x*))"]
-  ["binding restores"    "1"       "(do (def ^:dynamic *y* 1) (binding [*y* 9] nil) *y*)"])
+  ["binding restores"    "1"       "(do (def ^:dynamic *y* 1) (binding [*y* 9] nil) *y*)"]
+  ["var-set in binding"  "5"       "(do (def ^:dynamic *z* 1) (binding [*z* 0] (var-set (var *z*) 5) *z*))"])
 
 (defspec "namespaces / ns operations"
   ["in-ns switches"     "true"     "(do (in-ns 'my.ns) (symbol? 'x))"]