flipdim is slow/type inference issue for prod of vararg tuple

[simonster]

flipdim is much slower than I expected it to be, seemingly because of a type inference issue. code_typed shows M = prod(leading::(Int64...,)) (with no type), but I think this should actually be inferred, since code_typed(prod, ((Int64...),)) shows that return values of all applicable methods are Int64.