Add CompareAndSwap method

[erni27]

This PR adds CompareAndSwap method to both Cache and Sharded types.

Fixes: #45

[codecov-commenter]

Codecov Report

Merging #50 (39f8fe9) into master (c2f6d27) will increase coverage by 0.44%.
The diff coverage is 90.24%.

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@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   95.47%   95.91%   +0.44%     
==========================================
  Files           8        7       -1     
  Lines         796      833      +37     
==========================================
+ Hits          760      799      +39     
+ Misses         31       30       -1     
+ Partials        5        4       -1     

Files Changed	Coverage Δ
imcache.go	98.49% <90.24%> (+0.09%)	⬆️