core: start Phase 4 — move vary-meta and namespace-munge to the overlay

First Phase 4 batch: the host-coupled fns whose logic is pure composition of
existing core primitives, so no new jolt.host surface is needed. vary-meta is
just (with-meta obj (apply f (meta obj) args)); namespace-munge is a char map
over str. Both land in the 20-coll tier; the Janet core-* defns and bindings
are gone. Added namespace-munge spec cases and a vary-meta extra-args case.

The heavily host-coupled fns (atom/var/transient internals, arrays, proxy,
futures) stay in Janet pending a thin host-primitive expansion.

test/spec/metadata-spec.janet

@@ -7,6 +7,7 @@
   ["with-meta preserves value" "true" "(= [1 2 3] (with-meta [1 2 3] {:a 1}))"]
   ["with-meta on map"     "{:doc \"x\"}" "(meta (with-meta {:k 1} {:doc \"x\"}))"]
   ["vary-meta"            "{:a 2}"     "(meta (vary-meta (with-meta [1] {:a 1}) update :a inc))"]
+  ["vary-meta extra args" "{:a 1 :b 2}" "(meta (vary-meta (with-meta [1] {:a 1}) assoc :b 2))"]
   ["meta reader ^"        "{:tag :int}" "(meta ^{:tag :int} [1 2])"]
   ["with-meta on fn ok"   "true"       "(fn? (with-meta inc {:a 1}))"]
   ["with-meta nil clears" "nil"        "(meta (with-meta [1 2 3] nil))"])

test/spec/strings-spec.janet

@@ -48,3 +48,8 @@
   ["subs end past len"  :throws    "(subs \"abcde\" 1 6)"]
   ["subs nil start"     :throws    "(subs \"abcde\" nil 2)"]
   ["subs on nil"        :throws    "(subs nil 1 2)"])
+
+(defspec "string / namespace-munge"
+  ["hyphens to underscores" "\"a_b_c\"" "(namespace-munge \"a-b-c\")"]
+  ["from a symbol"          "\"foo_bar\"" "(namespace-munge (quote foo-bar))"]
+  ["no hyphens unchanged"   "\"ok\""    "(namespace-munge \"ok\")"])