fix: case composite constants, associative?/reversible?, update non-fn args, nth nil

- case: quote list literals (read as arrays) in constant position so a wrapped
  list ((a b c)) matches by value instead of being evaluated as a call; symbol
  constants already quoted. Vector/map/set constants already worked. case errors
  in the suite drop to 0 (60 pass).
- associative?: true only for vectors (pvec) and maps (phm/struct/sorted-map),
  not lists/tuples-from-seq-fns/lazy-seqs/sets.
- reversible?: true for vectors and sorted-map/sorted-set only.
- update: coerce f via as-fn so (update m k :kw)/(update m k a-set) work; extra
  args already handled.
- nth: (nth nil i)/(nth nil i default) returns nil/default instead of throwing.

clojure-test-suite pass 3649->3678, errors 122->105, clean files 44->46.
associative?/reversible? files now fully clean. spec: predicates + control/case.
jpm test green.

test/spec/predicates-spec.janet

@@ -34,6 +34,12 @@
   ["sequential? vector" "true"   "(sequential? [1])"]
   ["associative? map"   "true"   "(associative? {:a 1})"]
   ["associative? vec"   "true"   "(associative? [1])"]
+  ["associative? list"  "false"  "(associative? '(1 2))"]
+  ["associative? set"   "false"  "(associative? #{1})"]
+  ["reversible? vec"    "true"   "(reversible? [1 2])"]
+  ["reversible? list"   "false"  "(reversible? '(1 2))"]
+  ["reversible? smap"   "true"   "(reversible? (sorted-map :a 1))"]
+  ["reversible? hmap"   "false"  "(reversible? (hash-map :a 1))"]
   ["indexed? vector"    "true"   "(indexed? [1])"]
   ["counted? vector"    "true"   "(counted? [1])"])