`scan` is too complicated, suggestion for new iterator methods

[AngelicosPhosphoros]

@matklad

I am not even sure we need fold and try_fold (other methods with explicit state) -- for_each and try_for_each seem to be more rustic.

I think, fold is needed. for_each is too imperative for my taste and fold allows running functions without any state in them.

let result = iter.fold(initial, my_pure_function);
// vs
let mut result = initial;
iter.for_each(|x|result = my_pure_function(result, x));
let result = result; // to remove mutability

@scottmcm

You're absolutely correct that it's better to use for_each with an FnMut closure if you just need to mutate something, rather than using fold and threading it through all the calls -- not only is it simpler, it's faster too.

I think, it is more about missed optimization in rustc or LLVM or std.