Rename to ComplexExpression.

SubQuery means something specific in SQL and we don't want to confuse
people by reusing an existing term and giving it a new meaning. Instead,
let's call it a complex expression, which is more evocative of what it's
used for.

Move from Travis to GitHub actions.

Also, update our README with the latest info.

🎵 Now Playing: Really Over
🎵 Artist: Sub-Radio
🎵 Album: Past Selves

First public release.

After an extensive rewrite, lots of new tests being added, some
benchmarking, an actual README, and a few other tweaks, we're finally
ready for our first public release, making pan usable by people who are
not-us.