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.
023fe637ce