Provide a .windows type interface

It would be useful if you could either use .windows like you could in a regular iterator, windows may or may not be consuming (I would assume regular windows should be consuming like calling .next). Also the ability to peek at windows, a useful interface would be something like:
```
let mut iter = bytes.iter().peekable();

while let Some(window) = iter.peek_window::<4>() {
```
This would be pretty useful for parsers as one use case.

No file or test is named in the issue. Start by reading the existing iterator adapter's public API and tests, then define the consuming behavior and interface for windows and peek_window; done means the API and its semantics are unambiguous and covered by tests.

Written by the indexing model from the issue text.