fix nthrest empty-list semantics + if-let/when-let/if-some/when-some scoping

Two correctness bugs found while migrating:

- nthrest returned nil where Clojure returns () : for n>0 the walk yields
  (seq xs) wrapped in (or ... ()), so an exhausted/nil walk is () not nil
  (only n<=0 returns coll). Both the prior Janet impl and the first overlay
  port got this wrong. nthrest.cljc 13/1/0 -> 14/0/0.

- if-let/when-let/if-some/when-some leaked their binding into the else branch:
  they wrapped the whole if in (let* [name val] ...), so (let [x 5] (if-let
  [x nil] x x)) returned nil instead of 5. Fixed to bind a fresh temp around the
  if and rebind the name only inside the taken branch, matching Clojure.

conformance 218/218 x3, clojure-test-suite 3928 -> 3929.

jolt-core/clojure/core/20-coll.clj

@@ -122,13 +122,16 @@
   ([f] (completing f identity))
   ([f cf] (fn ([] (f)) ([x] (cf x)) ([x y] (f x y)))))
 
-;; Canonical loop form: short-circuits on an empty/nil coll before examining n
-;; (so (nthrest nil n) is nil without a number check), matching Clojure.
+;; Matches Clojure exactly: n<=0 returns coll unchanged; for n>0 the walk yields
+;; (seq xs), and an exhausted/nil walk falls back to () via (or ... ()) — so
+;; (nthrest nil 100) is () (not nil), while (nthrest nil 0) is nil.
 (defn nthrest [coll n]
-  (loop [n n xs coll]
-    (if (and (pos? n) (seq xs))
-      (recur (dec n) (rest xs))
-      xs)))
+  (if (pos? n)
+    (or (loop [n n xs coll]
+          (let [s (and (pos? n) (seq xs))]
+            (if s (recur (dec n) (rest s)) (seq xs))))
+        (list))
+    coll))
 
 (defn abs [x] (if (neg? x) (- 0 x) x))
 

src/jolt/core.janet

@@ -2160,16 +2160,21 @@
           {:jolt/type :symbol :ns nil :name "or__x"}
           @[{:jolt/type :symbol :ns nil :name "or"} ;(tuple/slice exprs 1)]]])))
 
+# if-let / when-let / if-some / when-some bind the name ONLY in the then/body
+# branch — the else branch must see the surrounding scope, not the binding (so
+# (let [x 5] (if-let [x nil] x x)) returns 5, like Clojure). Achieved with a fresh
+# temp around the if; the name is rebound to the temp inside the taken branch only.
+(defn- sym* [name] {:jolt/type :symbol :ns nil :name name})
+
 (defn core-if-let
   "Macro: (if-let [binding val-expr] then else?)"
   [bindings then-form & else-forms]
   (def form-sym (in bindings 0))
   (def val-form (in bindings 1))
-  @[{:jolt/type :symbol :ns nil :name "let*"}
-    @[form-sym val-form]
-    @[{:jolt/type :symbol :ns nil :name "if"}
-      form-sym
-      then-form
+  (def temp (gensym "if_let__"))
+  @[(sym* "let*") @[temp val-form]
+    @[(sym* "if") temp
+      @[(sym* "let*") @[form-sym temp] then-form]
       ;else-forms]])
 
 (defn core-when-let
@@ -2177,22 +2182,21 @@
   [bindings & body]
   (def form-sym (in bindings 0))
   (def val-form (in bindings 1))
-  @[{:jolt/type :symbol :ns nil :name "let*"}
-    @[form-sym val-form]
-    @[{:jolt/type :symbol :ns nil :name "when"}
-      form-sym
-      ;body]])
+  (def temp (gensym "when_let__"))
+  @[(sym* "let*") @[temp val-form]
+    @[(sym* "if") temp
+      @[(sym* "let*") @[form-sym temp] @[(sym* "do") ;body]]
+      nil]])
 
 (defn core-if-some
   "Macro: (if-some [binding val-expr] then else?)"
   [bindings then-form & else-forms]
   (def form-sym (in bindings 0))
   (def val-form (in bindings 1))
-  @[{:jolt/type :symbol :ns nil :name "let*"}
-    @[form-sym val-form]
-    @[{:jolt/type :symbol :ns nil :name "if"}
-      @[{:jolt/type :symbol :ns nil :name "some?"} form-sym]
-      then-form
+  (def temp (gensym "if_some__"))
+  @[(sym* "let*") @[temp val-form]
+    @[(sym* "if") @[(sym* "some?") temp]
+      @[(sym* "let*") @[form-sym temp] then-form]
       ;else-forms]])
 
 (defn core-when-some
@@ -2200,11 +2204,11 @@
   [bindings & body]
   (def form-sym (in bindings 0))
   (def val-form (in bindings 1))
-  @[{:jolt/type :symbol :ns nil :name "let*"}
-    @[form-sym val-form]
-    @[{:jolt/type :symbol :ns nil :name "when"}
-      @[{:jolt/type :symbol :ns nil :name "some?"} form-sym]
-      ;body]])
+  (def temp (gensym "when_some__"))
+  @[(sym* "let*") @[temp val-form]
+    @[(sym* "if") @[(sym* "some?") temp]
+      @[(sym* "let*") @[form-sym temp] @[(sym* "do") ;body]]
+      nil]])
 
 (defn core-doto
   "Macro: (doto obj (method args)...) → let obj, call methods, return obj"