Add sparse matrix support

[codecov[bot]]

Codecov Report

Merging #18 (f43f7e2) into master (9aa4f6b) will increase coverage by 1.25%. The diff coverage is 97.50%.

:exclamation: Current head f43f7e2 differs from pull request most recent head 1c15d1c. Consider uploading reports for the commit 1c15d1c to get more accurate results

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   79.33%   80.59%   +1.25%     
==========================================
  Files          12       13       +1     
  Lines         939      979      +40     
==========================================
+ Hits          745      789      +44     
+ Misses        194      190       -4     

Impacted Files	Coverage Δ
src/Pickle.jl	89.13% <ø> (ø)
src/sparse.jl	96.77% <96.77%> (ø)
src/np.jl	87.31% <100.00%> (+1.71%)	:arrow_up:
src/deserializer.jl	58.77% <0.00%> (+3.05%)	:arrow_up:

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[chengchingwen]

Thanks!