Add iterators (#32)

* Add iterators

- Iter returns front-to-back go iterator
- RIter returns back-to-front go iterator
- IterPopFront returns go iterator that removes items from front of Deque
- IterPopBack returns go iterator that removes items from back fo Deque

Using iterators to operate on sequences of items in Deque can avoid unnecessary range checks and multiple resizes. Iterators also allow Deque to be used with slices.Collect, slices.Sorted, etc.

* Show Iter working with slices.Sorted in test
* Describe iterators in README

Update README.md

Fix Clear not zeroing items when buffer full (#26)

* Fix Clear not zeroing items when buffer full

When the deque buffer is full Clear was not zeroing the items in it. This fixes that problem.

Additional changes:
- New out-of-range panic messages
- Minor document spelling and formatting fixes

Use generics to define Deque instance for a type (#23)

* Use generics to define Deque instance for a type
* Require go1.18

Add RIndex function

Update README.md

Update readme