ocaml: parser accepts if/match/let/fun as rhs of <- and :=

Previously `a.(i) <- if c then x else y` failed with
"unexpected token keyword if" because parse-binop-rhs called
parse-prefix for the rhs, which doesn't accept if/match/let/fun.

Real OCaml allows full expressions on the rhs of <-/:=. Fix:
special-case prec-1 ops in parse-binop-rhs to call parse-expr-no-seq
instead of parse-prefix. The recursive parse-binop-rhs with
min-prec restored after picks up any further chained <- (since both
ops are right-associative with no higher-prec binops above them).

Manacher baseline updated to use bare `if` on rhs of <-,
removing the parens workaround from iter 235. 607/607 regressions
remain clean.

lib/ocaml/baseline/manacher.ml

@@ -13,7 +13,7 @@ let manacher s =
     if i < !right then begin
       let v = !right - i in
       let pm = p.(mirror) in
-      p.(i) <- (if pm < v then pm else v)
+      p.(i) <- if pm < v then pm else v
     end;
     while i + p.(i) + 1 < m && i - p.(i) - 1 >= 0
           && t.(i + p.(i) + 1) = t.(i - p.(i) - 1) do

lib/ocaml/parser.sx

@@ -741,16 +741,28 @@
                       (else
                         (begin
                           (advance-tok!)
-                          (let
-                            ((rhs (parse-prefix))
-                              (next-min
-                                (if
-                                  (ocaml-binop-right? op)
-                                  prec
-                                  (+ prec 1))))
-                            (begin
-                              (set! rhs (parse-binop-rhs rhs next-min))
-                              (parse-binop-rhs (list :op op lhs rhs) min-prec))))))))))))
+                          ;; For `<-` and `:=`, the rhs is at expression
+                          ;; level — accept `if/match/let/fun/...` keywords
+                          ;; on the right (real OCaml does). Otherwise,
+                          ;; fall back to the standard prefix-then-binop
+                          ;; chain.
+                          (cond
+                            ((or (= op "<-") (= op ":="))
+                              (let ((rhs (parse-expr-no-seq)))
+                                (parse-binop-rhs
+                                  (list :op op lhs rhs) min-prec)))
+                            (else
+                              (let
+                                ((rhs (parse-prefix))
+                                  (next-min
+                                    (if
+                                      (ocaml-binop-right? op)
+                                      prec
+                                      (+ prec 1))))
+                                (begin
+                                  (set! rhs (parse-binop-rhs rhs next-min))
+                                  (parse-binop-rhs
+                                    (list :op op lhs rhs) min-prec))))))))))))))
         (define
           parse-binary
           (fn