ocaml: phase 2+3 'when' guard in try/with (+3 tests, 467 total)

parse-try now consumes optional 'when GUARD-EXPR' before -> and emits
(:case-when PAT GUARD BODY). Eval try clause loop dispatches on case /
case-when and falls through on guard false — same semantics as match.

Examples:
  try raise (E 5) with | E n when n > 0 -> n | _ -> 0   = 5
  try raise (E (-3)) with | E n when n > 0 -> n | _ -> 0 = 0
  try raise (E 5) with | E n when n > 100 -> n | E n -> n + 1000  = 1005

lib/ocaml/parser.sx

@@ -773,11 +773,18 @@
                   (begin
                     (define one
                       (fn ()
-                        (let ((p (parse-pattern)))
+                        (let ((p (parse-pattern)) (guard nil))
                           (begin
+                            (when (at-kw? "when")
+                              (begin (advance-tok!)
+                                     (set! guard (parse-expr-no-seq))))
                             (consume! "op" "->")
                             (let ((body (parse-expr)))
-                              (append! cases (list :case p body)))))))
+                              (cond
+                                ((= guard nil)
+                                  (append! cases (list :case p body)))
+                                (else
+                                  (append! cases (list :case-when p guard body)))))))))
                     (one)
                     (define loop
                       (fn ()