Switch from ailing TravisCI to trendy GitHub Actions

[exarkun]

TravisCI is in bad shape these days and not seeming to get any better.

This deletes the TravisCI configuration and adds a GitHub Actions CI configuration.

Somehow the TravisCI configuration worked with a number of organizational problems and legacy choices in the repository. I didn't feel like doing extra work to make GitHub Actions work with that situation. Instead, I fixed those problems and choices alongside these CI configuration changes.

Separating these two things into different PRs doesn't make sense because there's no reason to try to update the TravisCI configuration to work with the fixes and then immediately abandon TravisCI.

[coveralls]

Coverage decreased (-1.08%) to 91.962% when pulling 7ecb8cd on github-actions into e84abc9 on master.

[exarkun]

Hard to say why coveralls is reporting a coverage decrease. It reports the coverage is lower but it also reports an empty list of coverage changes. Looking at different builds and trying to see the change manually is difficult. Different builds don't even render the same, there is no stable file ordering (even "sort by filename" produces different results on different builds!). Older builds seem to be missing branch coverage even though the travis configuration was explicitly passing --branch.

So presumably the change is an artifact of how coverage is being measured and/or reported. Maybe we are missing the measurement of some coverage that was being covered before or maybe the measurement before was bogus in some way and has accidentally been fixed. I don't know. I am probably not going to try diving through the details too much more to understand this.

[mithrandi]

This all seems good enough to me; the coverage stuff is probably bogus due to the last build being so old. We can always fiddle more once we have green builds again.