Configure 'hlint'

[chshersh]

To have a best-in-class dev environment, it would be great to configure HLint for Iris and add a CI job for this.

Specifically:

• [x] Use the appropriate HLint config and store it in the repo
• [x] Fix or ignore HLint warnings
• [x] Add HLint CI job
• [ ] Document hlint usage and requirements