Easy to use abstinential (slippery), N-peekable and rewindable wrapper over an iterator. (Nightly Rust)

Example:
```rs
use slippery_iter::{SlipperyIter, SlipperyIterator, IntoSlipperyIterator};

fn main() {
  let a = 0..=10;
  let mut b = SlipperyIter::from(a);
  println!("{:?}", b.peek_forward());
  println!("{:?}", b.peek_forward_many::<8>());
  println!("{:?}", b.consume_many::<6, 0>());
  println!("{:?}", b.peek_forward());
  println!("{:?}", b.peek_forward_many::<8>());
}
```