Phase 3 batch 2 (jolt-1j0), 5 user-facing macros as defmacro + syntax-quote.
The conditional-binding macros use the auto-gensym temp# idiom so the name binds
only in the taken branch (the else/empty branch sees the outer scope, carrying
forward the earlier scope fix). when-let stays in Janet for now — 20-coll uses
it, so it must remain available before the macro tier loads.
Note: gensym in a macro body resolves to Janet's 0-arity builtin, so explicit
(gensym "prefix") fails — use template auto-gensym (foo#) instead.
conformance 228/228 x3, full suite green (incl. the if-let scope regression spec,
now exercising the overlay macro), clojure-test-suite 3930, bench A/B flat.
e611034550