core: port when-let to the overlay — finishes the Phase 3 macro migration

Last core macro still in Janet. Goes in 00-syntax (not 30-macros with its
if-let/if-some/when-some siblings) because 20-coll's not-empty uses it and
20-coll loads before 30. Via `let` so the binding form may destructure,
matching Clojure — the old Janet version used let* and wouldn't.

Drops core-when-let + the sym* helper + the intern entry, and empties
core-macro-names (no seed binding is a macro anymore).

jolt-core/clojure/core/30-macros.clj

@@ -25,6 +25,8 @@
     `(let [temp# ~tst]
        (if temp# (let [~form temp#] ~then) ~else))))
 
+;; when-let lives in 00-syntax (not here): 20-coll uses it, which loads before this tier.
+
 (defmacro if-some [bindings then & [else]]
   (let [form (bindings 0) tst (bindings 1)]
     `(let [temp# ~tst]