Use Github Actions to run tests

[dolfinus]

Just like #90, but instead of CircleCI it's better to use Github Actions.
Github Actions supports matrix builds (unlike Circle CI), and also allows to build really impressive CI/CD pipelines and PR/issues automatization. Here you can find more details about features.
Projects like ecto and phoenix are already moved to Github Actions.

Here you can find an example of successfully executed workflow: https://github.com/dolfinus/paginator/actions/runs/610852525

[dolfinus]

@sgerrand @jbernardo95 @msz What do you think about this PR?

[sgerrand]

Thank you for your contribution! These changes look good overall. I've added a few suggestions.

One thing missing is a link to this workflow running successfully. Would you please add this, either in the description or as a comment, to make it easier for other reviewers.

[sgerrand]

I think one other thing which needs to be done before this can be merged is enabling GitHub Actions on this repository and ensuring that at least 2-3 runs of this workflow complete successfully.

[sgerrand]

Github Actions supports matrix builds (unlike Circle CI)

Note that CircleCI has supported matrices in their builds since April 2020. I only became aware of this myself late last year.

[dolfinus]

Thank your for review!

I've added a link to a successfully executed workflow into PR description.

[sgerrand]

LGTM. 👍