core: move destructure from the Janet seed to the Clojure overlay

Port clojure.core/destructure into jolt-core/clojure/core/00-syntax.clj
and drop core-destructure (plus the d-* helpers) from core.janet. The
expander is now self-hosted, shrinking the seed per the jolt-1j0 epic.

It's def+fn* rather than defn because defn isn't defined yet at that
point in the syntax tier. Only the let macro consumes its output, so let
now splices [~@(destructure bindings)] to keep a tuple binding form.

Also fix a gap the old Janet version papered over via Janet's
keyword->string: :keys accepts keyword elements ({:keys [:major :minor]}),
so use name/namespace for the local + ns instead of str (which keeps the
colon). This was breaking sci's impl/namespaces.cljc. Added spec cases.

test/spec/destructuring-spec.janet

@@ -35,7 +35,10 @@
   [":strs"              "7"      "(let [{:strs [a]} {\"a\" 7}] a)"]
   [":syms"              "8"      "(let [{:syms [a]} {(quote a) 8}] a)"]
   ["namespaced :keys"   "3"      "(let [{:keys [x/y]} {:x/y 3}] y)"]
-  ["namespaced :syms"   "4"      "(let [{:syms [p/q]} {(quote p/q) 4}] q)"])
+  ["namespaced :syms"   "4"      "(let [{:syms [p/q]} {(quote p/q) 4}] q)"]
+  # :keys also accepts keyword elements ({:keys [:a :b]}), binding bare locals.
+  ["keyword :keys"      "3"      "(let [{:keys [:a :b]} {:a 1 :b 2}] (+ a b))"]
+  ["keyword :keys ns"   "3"      "(let [{:keys [:x/y]} {:x/y 3}] y)"])
 
 (defspec "destructure / keyword args (& {:keys})"
   ["fn kwargs"          "[1 2]"  "(do (defn f [& {:keys [a b]}] [a b]) (f :a 1 :b 2))"]