ADR 020 - Protocol Buffer Transaction Encoding

[alexanderbez]

/cc @aaronc @jackzampolin @marbar3778

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5757 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5757   +/-   ##
=======================================
  Coverage   35.91%   35.91%           
=======================================
  Files         341      341           
  Lines       34745    34745           
=======================================
  Hits        12478    12478           
  Misses      21023    21023           
  Partials     1244     1244