JVM-semantics fixes and small cleanups

- take-last / drop-last return seqs, not vectors: take-last wraps in seq; drop-last
  is the JVM (map (fn [x _] x) coll (drop n coll)) form (lazy, () when empty).
- cycle is lazy ((lazy-seq (concat coll (cycle coll)))) so it no longer counts its
  argument and terminates on a lazy/infinite input.
- fold's foldable-call catch uses :default, matching the rest of jolt-core and
  also catching a raw host condition from a folding primitive.
- alts! rejects non-channel ports with a clear error (put specs / :default are
  unsupported) instead of crashing inside ac-poll!.
- Misc: drop the unreachable second getCause clause; jolt-nth on a string raises
  'nth "index out of bounds" like the vector branch; name the inline fixpoint cap;
  bld-sh-capture rejoins lines with newlines; clarify a couple of comments.

host/chez/collections.ss

@@ -243,7 +243,8 @@
        (cond ((pvec? coll) (let ((v (pvec-v coll)))
                              (if (and (fx>=? i 0) (fx<? i (vector-length v))) (vector-ref v i)
                                  (error 'nth "index out of bounds"))))
-             ((string? coll) (string-ref coll i))
+             ((string? coll) (if (and (fx>=? i 0) (fx<? i (string-length coll))) (string-ref coll i)
+                                 (error 'nth "index out of bounds")))
              ((or (cseq? coll) (empty-list-t? coll)) (seq-nth coll i #f jolt-nil))
              (else (error 'nth "unsupported collection")))))
     ((coll i d)