Add an Iterator for LruCache

[jeromefroe]

Add an Iter struct for iterating over the elements in a LruCache.

[codecov-io]

Codecov Report

Merging #20 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
- Coverage    98.6%   98.59%   -0.01%     
==========================================
  Files           1        1              
  Lines         501      499       -2     
==========================================
- Hits          494      492       -2     
  Misses          7        7

Impacted Files	Coverage Δ
src/lib.rs	98.59% <100%> (-0.01%)	⬇️

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[rohitjoshi]

👍 Really appreciate. I will try this out. Thanks.