Add linters to the project

[ingydotnet]

Once we decide on our rules for #3 let's add linters for the various kinds of files used in the repo.

[banjoh]

https://github.com/golangci/golangci-lint is widely used. Once we get to deciding what direction to take, we can consider using it. Its a runner for many linters implemented as plugins, and can be configured to a small set.

[zostay]

o/ Ingy! I have a lot of projects setup to use golangci-lint and can contribute boilerplate. I just ran my usual configuration against the source and got 149 issues to start. For example, it found a lot of unused internal parameters, some formatting and spelling issues, some unchecked integer conversions, a bunch of switch statements that aren't exhaustive (but probably don't need to be), a couple unchecked errors, and a few cases of errors being compared directly rather than using errors.As or errors.Is.

I'm happy to provide a PR for this and include a github action that will automatically lint pushes or PRs. I like going as far as having gofumpt enforced on my own projects, but I'm happy to contribute my full suite with fixes or something that is all but empty to provide a starting point.

[ingydotnet]

@zostay Can you group categories of issues into their own PRs to make them easier to approve?

Feel free to start submitting PRs for this.