Added presubmit script distilled from that in trillian

[mhutchinson]

Removed some of the flags and branches that weren't needed, such as code
generation. This isn't hooked into CI yet, but at least developers can
run ./scripts/presubmit.sh before submitting in the meantime.

[codecov-commenter]

Codecov Report

Merging #29 (0d36fe9) into main (ff6e921) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #29   +/-   ##
=======================================
  Coverage   88.93%   88.93%           
=======================================
  Files           7        7           
  Lines         497      497           
=======================================
  Hits          442      442           
  Misses         50       50           
  Partials        5        5           

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[pav-kv]

Do you reckon this script is github-actionable?

[mhutchinson]

Do you reckon this script is github-actionable?

I'm planning on making it consistent with the other repos, which have GCB run a docker container that performs all of the checks. If we foresee only ever wanting the linter (but we'd need to drop the licence check from presubmit) then maybe we could look at https://github.com/golangci/golangci-lint-action.

[pav-kv]

Consistency across repos is the best thing, SGTM.

I like the simplicity of actions, it worked well for this merkle repo for adding quick easy checks. I don't know if it can fully replace GCB though. Maybe the latter can be used for more involved integration tests.