core: variadic bit ops, set? covers sorted sets, if rejects extra forms

Three canonical-conformance fixes from the post-shrink batch:

- bit-and/bit-or/bit-xor/bit-and-not get Clojure's variadic arities as
  20-coll shells folding the binary host ops (now __bit-* seams). 2-arg call
  sites still compile to the native janet op via the backend's native-ops
  table. The passes.clj constant-fold table now names the seams — the public
  fns are overlay and don't exist when the compiler loads (this briefly broke
  every compile-mode init).

- core-set? recognizes the :jolt/sorted-set representation (jolt-dpn):
  (set? (sorted-set 1)) was false, and ifn? on sorted sets inherited the bug.

- (if) / (if test) / (if test then else extra) throw in both the analyzer
  and the interpreter — spec 03-special-forms X1, now marked verified.

Suite 4704 -> 4706; bench and the greeter example benchmark are flat.

test/spec/forms-spec.janet

@@ -81,3 +81,12 @@
   ["call literal fn" "1"      "((fn* [] 1))"]
   ["call nested"     "6"      "(+ ((fn* [] 1)) ((fn* [] 2)) ((fn* [] 3)))"]
   ["call nil"        :throws  "(nil)"])
+
+# if takes two or three argument forms — anything else is a compile-time
+# error (spec 03-special-forms X1).
+(defspec "forms / if arity (X1)"
+  ["bare if throws"      :throws "(if)"]
+  ["one-arg if throws"   :throws "(if true)"]
+  ["four-arg if throws"  :throws "(if true 1 2 3)"]
+  ["two-arg if ok"       "nil"   "(if false 1)"]
+  ["three-arg if ok"     "2"     "(if false 1 2)"])