Fix four bugs surfaced by the commonmark-app example

- regex: bounded quantifiers {n,m} no longer expand exponentially. The
  desugaring inlined the continuation per optional level, doubling it each
  time, so {0,61} built a PEG the compiler expanded to ~2^61 nodes and hung.
  Each level is now its own grammar rule referenced by name (jolt-3xur).

- strings are a seqable of chars for vec/set/into, matching seq. They went
  through realize-for-iteration, which had no string case, so into/set got
  raw bytes (code points) and set threw; vec used string/from-bytes (1-char
  strings). realize-for-iteration now maps make-char over the bytes, and
  core-vec matches (jolt-dl4s).

- clojure.string/split takes Java Pattern.split limit semantics: negative
  keeps trailing empties, 0/omitted drops them (a no-match result stays
  [input]), positive caps the part count with the remainder unsplit. It used
  to delegate the limit to take, so a negative limit returned [] and the
  2-arg form never trimmed trailing empties (jolt-ik3a).

- System/exit is registered (maps to os/exit), so (System/exit n) works
  (jolt-w2wf).

test/spec/regex-spec.janet

@@ -62,3 +62,13 @@
   [".matches partial -> false" "false" "(.matches \"abcd\" \"a.c\")"]
   [".replaceAll regex"   "\"a-b-c\"" "(.replaceAll \"a_b_c\" \"_\" \"-\")"]
   [".replaceFirst regex" "\"a-b_c\"" "(.replaceFirst \"a_b_c\" \"_\" \"-\")"])
+
+(defspec "regex / bounded quantifiers (jolt-3xur)"
+  ["exact {n}"        "\"aaa\""  "(re-matches #\"a{3}\" \"aaa\")"]
+  ["range {n,m} max"  "\"aaaa\"" "(re-find #\"a{2,4}\" \"aaaaa\")"]
+  ["zero lower {0,n}" "\"aa\""   "(re-find #\"a{0,3}\" \"aa\")"]
+  ["{n,} unbounded"   "\"aaaa\"" "(re-find #\"a{2,}\" \"aaaa\")"]
+  # nested bounded quantifiers must not blow up the PEG compiler — this used to
+  # expand to ~2^61 nodes and hang at compile time.
+  ["nested bounds compile" "true"
+   "(boolean (re-matches #\"[a-z](?:[a-z]{0,61}[a-z])?(?:-[a-z]{0,61}[a-z])*\" \"abc-def\"))"])