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.
giles
4fdf6980da