core: Phase 4 — move reduce-kv to the overlay, fixing the vector case

reduce-kv is pure over reduce/keys/get/nth, so it moves to 20-coll with no
host surface. Both branches fold through reduce, which fixes two latent bugs
in the Janet version: on a vector it saw the pvec as a table and folded over
its internal keys instead of the indices, and it ignored reduced entirely.
nil folds to init. Added vector-index, reduced, and nil spec cases plus a
3-mode conformance case for the vector fix.

test/integration/conformance-test.janet

@@ -108,6 +108,7 @@
    ["subvec"          "[2 3]"         "(subvec [1 2 3 4 5] 1 3)"]
    ["subvec to-end"   "[3 4 5]"       "(subvec [1 2 3 4 5] 2)"]
    ["reduce-kv"       "{:a 2 :b 3}"   "(reduce-kv (fn [m k v] (assoc m k (inc v))) {} {:a 1 :b 2})"]
+   ["reduce-kv vector idx" "(quote ([0 :a] [1 :b]))" "(reduce-kv (fn [a i v] (conj a [i v])) [] [:a :b])"]
 
    ### ---- iterating maps yields entries ----
    ["map over map"      "true"  "(= #{1 2} (set (map val {:a 1 :b 2})))"]

test/spec/sequences-spec.janet

@@ -56,6 +56,9 @@
   ["reduce single no init"  "5"           "(reduce + [5])"]
   ["reduced short-circuits" "3"           "(reduce (fn [a x] (if (> a 2) (reduced a) (+ a x))) 0 [1 2 3 4 5])"]
   ["reduce-kv"              "6"           "(reduce-kv (fn [a k v] (+ a v)) 0 {:a 1 :b 2 :c 3})"]
+  ["reduce-kv on vector"    "[[0 :a] [1 :b]]" "(reduce-kv (fn [a i v] (conj a [i v])) [] [:a :b])"]
+  ["reduce-kv honors reduced" "[:a]"      "(reduce-kv (fn [a i v] (if (= i 1) (reduced a) (conj a v))) [] [:a :b :c])"]
+  ["reduce-kv on nil"       "0"           "(reduce-kv (fn [a k v] (+ a v)) 0 nil)"]
   ["reductions"             "[1 3 6]"     "(reductions + [1 2 3])"]
   ["mapcat"                 "[1 1 2 2]"   "(mapcat (fn [x] [x x]) [1 2])"]
   ["keep"                   "[1 3]"       "(keep (fn [x] (if (odd? x) x nil)) [1 2 3 4])"]