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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

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