Macro iter 

macro_rules! iter {
    ($e:expr) => { ... };
    ($e:expr, $min_len:expr) => { ... };
}

Returns either a regular or a parallel iterator depending on whether concurrent feature is enabled.

When concurrent feature is enabled, creates a parallel iterator; otherwise, creates a regular iterator. Optionally, min_length can be used to specify the minimum length of iterator to be processed in each thread.

Adapted from: https://github.com/arkworks-rs/utils/blob/master/src/lib.rs