core: move the pure-leaf fns to the overlay; memfn is a real macro now

Round 3 of the seed shrink. To the overlay: identity, constantly, neg?,
even?, odd? (20-coll, ahead of their first in-tier uses), not= and unreduced
(00-syntax — the kernel and seq tiers use them), ==, ensure-reduced,
halt-when, parse-boolean, parse-uuid, newline, seque, array-seq, to-array-2d,
and the masking unchecked-byte/short/char/float/double coercions. parse-uuid
validates via re-matches over a new __make-uuid host binding (overlay source
can't write :jolt/type map literals). memfn moves to 30-macros as a working
macro over the .method call sugar instead of a fn that throws.

Behavior fixes toward Clojure, each with spec rows: == now throws on
non-numbers instead of comparing them, and halt-when is the canonical
::halt-map version (the halt value replaces the whole reduction result, no
double completion). list? and map-entry? stay in the seed — both are
representation-coupled (plist/tuple checks).

clojure-test-suite goes 4701 -> 4700: update.cljc expects
(update {:k 1} :k identity 1 2 3 4) to throw an arity error, and jolt fns
don't enforce fixed arity anywhere (pre-existing, language-wide — the seed's
Janet identity threw natively). Filed as jolt-6xn; fixing it should flip
several suite rows at once.

test/spec/numbers-spec.janet

@@ -192,3 +192,15 @@
   ["num passes a number through" "5"   "(num 5)"]
   ["num on a double"        "5.5" "(num 5.5)"]
   ["num throws on non-number" :throws "(num \"x\")"])
+
+# == is numeric equality: single arg is trivially true (Clojure's 1-arity
+# never inspects the value); 2+ args must be numbers.
+(defspec "numbers / == numeric equality"
+  ["== single arg"          "true"  "(== :a)"]
+  ["== equal"               "true"  "(== 2 2)"]
+  ["== unequal"             "false" "(== 2 3)"]
+  ["== chained"             "true"  "(== 2 2 2)"]
+  ["== chained unequal"     "false" "(== 2 2 3)"]
+  ["== int and double"      "true"  "(== 1 1.0)"]
+  ["== throws on non-number" :throws "(== 1 :a)"]
+  ["== throws on two keywords" :throws "(== :a :a)"])