core: move for to the syntax tier (and fix multi-group :when)

Port of core-for: desugar a comprehension to nested map/mapcat over the binding
colls, :let -> let*, :while -> take-while on the coll. Lives in 00-syntax because
doseq (already there) expands to it; the macro body uses only kernel/seed fns so it
runs at analyzer-build time. doseq no longer depends on a Janet macro.

Fixed a latent bug the Janet macro had: :when wrapped the inner form in (list ...)
unconditionally, but for an outer binding group the inner form is already a seq, so
mapcat produced a seq-of-seqs instead of flattening. e.g.
  (for [x [0 1] :when (odd? x) y [:a :b]] [x y])
gave ((... ...)) instead of ([1 :a] [1 :b]). Now the (list ...) wrap is only applied
to the last group's scalar body; outer groups contribute their seq directly. Added
:let+:when, multi-group :when, and destructuring regression cases.

conformance 228x3, fixpoint, clojure-test-suite 3930, full suite green, bench flat.

test/spec/control-flow-spec.janet

@@ -68,6 +68,9 @@
   ["for :when"          "[0 2 4]" "(for [x (range 6) :when (even? x)] x)"]
   ["for :while"         "[0 1 2]" "(for [x (range 10) :while (< x 3)] x)"]
   ["for :let"           "[0 1 4]" "(for [x (range 3) :let [sq (* x x)]] sq)"]
+  ["for :let+:when"     "[4 6 8]" "(for [x (range 5) :let [y (* x 2)] :when (> y 3)] y)"]
+  ["for multi :when"    "[[1 :a] [1 :b]]" "(for [x [0 1] :when (odd? x) y [:a :b]] [x y])"]
+  ["for destructure"    "[3 7]"   "(for [[a b] [[1 2] [3 4]]] (+ a b))"]
   ["doseq side-effect"  "6"      "(let [a (atom 0)] (doseq [x [1 2 3]] (swap! a (fn [v] (+ v x)))) @a)"]
   ["doseq nested"       "4"      "(let [c (atom 0)] (doseq [x [1 2] y [10 20]] (swap! c inc)) @c)"]
   ["doseq :when"        "[1 3]"  "(let [a (atom [])] (doseq [x [1 2 3] :when (odd? x)] (swap! a conj x)) @a)"]