PR: Replace Travis CI with GitHub Actions issue #230

[nelsonic]

Adding GitHub Actions to this repo has been an excellent opportunity to significantly reduce the amount of noise in the /examples and cut out most of the advanced examples that 99.9% of people will never read.
So I've taken this opportunity to delete the code. All of that sample code comes from http://www.pragmaticprogrammer.com/titles/elixir13 (404) at the time of writing ... 🙄
The newer version is: https://pragprog.com/titles/elixir16/programming-elixir-1-6/
and a basic Google search will show you the PDF: google.com/search?q=Programming+Elixir+1.6+pdf and thus the code.
No need to duplicate it here.

Note: I've kept the more beginner-friendly examples for people to get a feel for the syntax. 🔰

Other than that, the PR is focussed on adding a test for a sample set of functions and adding Codecov.
Though I have not added badges at this time because the one that I care about is "broken": dwyl/hits#306

[asntc]

LGTM.