Misc tidy up

[JOT85]

I'm about to make a couple of PRs but, before that, I've spent a bit of time tidying a few things up. These are only minor things, including:

• use std::fs::read_to_string function,
• use lines method on a BufReader,
• use iterators directly rather than collecting everything in to a vec (removes a bunch of allocation),
• add a couple of debug_asserts,
• a little bit of general formatting (mixed tabs -> all spaces, correct indentation levels etc),
• fixing clippy lints.

I've also added /usr/lib/os-release as a fallback to /etc/os-release as specified in the man page.

Thanks for your time :)

[JOT85]

Merge conflict from #78 resolved :)