Export concrete types

[erni27]

This PR exports concrete types for non-sharded cache (Cache) and sharded cache (Sharded).

Changes include:

• Rename Cache interface to Imcache
• Export a non-sharded cache type Cache
• Make Cache zero value ready to use
• Export a sharded cache type Sharded
• Update factory functions to return concrete types instead of interface

[codecov-commenter]

Codecov Report

Merging #22 (324ce92) into master (c109d14) will increase coverage by 0.03%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   97.45%   97.49%   +0.03%     
==========================================
  Files           8        8              
  Lines         393      399       +6     
==========================================
+ Hits          383      389       +6     
  Misses         10       10              

Impacted Files	Coverage Δ
cache.go	100.00% <100.00%> (ø)
option.go	100.00% <100.00%> (ø)

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