Fix five bugs surfaced by commonmark-app (jolt-3xur, dl4s, ik3a, w2wf, xtss) (#158)

* 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).

* regex: single-digit backreferences \1..\9 (jolt-xtss)

\1..\9 now match the text captured by the corresponding group, so
patterns like ([-*_])\1\1 or (\w+) \1 work. The parser records which
groups are referenced; a referenced group additionally captures its text
under a tag and the backref compiles to a PEG (backmatch). Only referenced
groups change — they match possessively (the CPS-over-possessive-PEG engine
can't backtrack into a tagged capture), so backtracking back into a
backreferenced group isn't supported (rare). Unreferenced groups keep full
backtracking and position-based result capture unchanged.

---------

Co-authored-by: Yogthos <yogthos@gmail.com>

test/spec/strings-spec.janet

@@ -20,6 +20,22 @@
   ["pr-str of a defn"   "\"#'user/gg\"" "(pr-str (defn gg [x] x))"]
   ["seq of string"      "[\\a \\b]"  "(seq \"ab\")"])
 
+(defspec "string as a seqable of chars (jolt-dl4s)"
+  ["vec of string"       "[\\a \\b]" "(vec \"ab\")"]
+  ["into [] of string"   "[\\a \\b]" "(into [] \"ab\")"]
+  ["set of string"       "true"      "(= #{\\a \\b} (set \"ab\"))"]
+  ["into #{} of string"  "true"      "(= #{\\a \\b} (into #{} \"ab\"))"]
+  ["set dedups chars"    "2"         "(count (set \"aab\"))"]
+  ["mapv over string"    "[\\a \\b]" "(mapv identity \"ab\")"])
+
+(defspec "clojure.string / split limit (jolt-ik3a)"
+  ["neg keeps trailing"  "[\"a\" \"\" \"\"]" "(do (require (quote [clojure.string :as s])) (s/split \"a,,\" #\",\" -1))"]
+  ["zero trims trailing" "[\"a\"]"           "(do (require (quote [clojure.string :as s])) (s/split \"a,,\" #\",\" 0))"]
+  ["omitted trims"       "[\"a\"]"           "(do (require (quote [clojure.string :as s])) (s/split \"a,,\" #\",\"))"]
+  ["positive caps parts" "[\"a\" \"b,c\"]"   "(do (require (quote [clojure.string :as s])) (s/split \"a,b,c\" #\",\" 2))"]
+  ["empty string"        "[\"\"]"            "(do (require (quote [clojure.string :as s])) (s/split \"\" #\",\"))"]
+  ["interior empties kept" "[\"a\" \"\" \"b\"]" "(do (require (quote [clojure.string :as s])) (s/split \"a,,b\" #\",\"))"])
+
 (defspec "clojure.string"
   ["join"               "\"a,b,c\""  "(do (require (quote [clojure.string :as s])) (s/join \",\" [\"a\" \"b\" \"c\"]))"]
   ["join no sep"        "\"abc\""    "(do (require (quote [clojure.string :as s])) (s/join [\"a\" \"b\" \"c\"]))"]