Add GitHub Actions CI

[iamed2]

TODO:

• [ ] Remove Travis CI
• [ ] Change badges (Travis -> GItHub, Coveralls -> Codecov)

I have Codecov in here because the Julia Actions support for Coveralls seemed less mature and also seemed like it might require access to secrets, which I do not have on this repo.

I only test on Linux because I don't expect there to be OS differences, but include x86 because of how important the UInt alias is to hashing. I also bumped the minimum tested version to LTS.

Closes #28

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@f643bdd). Click here to learn what that means. The diff coverage is n/a.

:mega: This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff            @@
##             master      #30   +/-   ##
=========================================
  Coverage          ?   97.36%           
=========================================
  Files             ?        1           
  Lines             ?       38           
  Branches          ?        0           
=========================================
  Hits              ?       37           
  Misses            ?        1           
  Partials          ?        0           

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[gafter]

Done in #32.