Add deduplication

[mdekstrand]

This adds the V2 binpickle format with deduplication (closes #11).

[codecov]

Codecov Report

Merging #15 (dd13c1f) into master (24e96a0) will decrease coverage by 1.55%.
The diff coverage is 89.59%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
- Coverage   96.08%   94.52%   -1.56%     
==========================================
  Files          12       13       +1     
  Lines         460      603     +143     
==========================================
+ Hits          442      570     +128     
- Misses         18       33      +15     

Impacted Files	Coverage Δ
binpickle/repickle.py	84.78% <84.78%> (ø)
binpickle/format.py	89.92% <88.76%> (-2.67%)	⬇️
binpickle/read.py	95.45% <92.85%> (+2.35%)	⬆️
binpickle/write.py	97.63% <100.00%> (+0.22%)	⬆️

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