Add Set method

[vejnar]

This is adding the Set() method with the same intent as At(): to be able to use deque as a more general purpose circular buffer (similar to deque in C++).
Please comment and/or merge.

The request removed the Copy method from #8 .

[codecov-io]

Codecov Report

Merging #9 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #9   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         102    106    +4     
=====================================
+ Hits          102    106    +4

Impacted Files	Coverage Δ
deque.go	100% <100%> (ø)	⬆️

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