Add BinaryMarshaler BinaryUnmarshaler implementation

[smallhive]

Closes #47

[codecov]

Codecov Report

Attention: Patch coverage is 89.65517% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 49.64%. Comparing base (9716b39) to head (2b1401d).
Report is 6 commits behind head on master.

Files	Patch %	Lines
tz/digest.go	89.28%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   47.56%   49.64%   +2.07%     
==========================================
  Files          10       10              
  Lines         534      562      +28     
==========================================
+ Hits          254      279      +25     
- Misses        260      262       +2     
- Partials       20       21       +1     

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