Add linter and CI

[mihaitodor]

Thought to add some linting and CI to this one too.

~I guess 93f81fa might have caused trouble in some cases, because the underlying data of slices is passed by reference.~ On second thought, after playing with some similar code a bit, I think the compiler is smart enough to do the right thing. I can revert to the original implementation and add a linter exception if you prefer that.

Also, there are a bunch of functions called in the tests which aren't checked for errors and I ignored them for now. Happy to add those checks if you don't mind the extra verbosity.