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