[WIP] Replace panic! and unreachable! with internal_error!

[matssigge]

I've gone through the "low risk" packages listed in #2046 and replaced panic! and unreachable! with internal_error! instead. For panic!, I've just replaced the name and haven't changed any parameters. For unreachable, there are two different cases:

1. A call with no parameters, i.e. unreachable!(). In this case I replaced it with internal_error!("unreachable"), so that it's still clear that this should never happen.
2. A call with an error string, e.g. unreachable!("Any other pattern should have given a parse error"). In this case, I've added "unreachable: " at the beginning of the string, again to make it clear that the code isn't expected to ever run.

I just realized that I haven't used user_error! at all, so I will go through previous uses of panic! and try to determine where user_error! is better than internal_error!.

[matssigge]

I had several attempts at this which all failed because I didn't manage to run the test suite locally. Since I've realized I don't have time to spend on this right now, I'm closing the PR again.

[JanCVanB]

@matssigge Thank you for attempting this! Would you like any help running the test suite locally? Our GitHub-hosted CI tests have also gotten better this year, if that's sufficient.

[matssigge]

@JanCVanB I'm sorry, but I don't have the time to work on this right now. I was between projects at the beginning of the year, giving me some free time, but since February I'm working full time, and now I don't have the bandwidth.

[JanCVanB]

@matssigge That's okay, congratulations on your full-time work! I hope that your and Roc's paths cross again someday 🙂