project modernizations

[tayloraswift]

i used to think (in 2021) that this library was the pinnacle of accumulated swift experience… now all i see is terrible practices and overdue reforms…

by my personal standards today i think this library still needs a lot more work, but these changes should get us a long ways there:

• adopt SPM-friendly directory structure
• split up obscenely large files and organize by type instead of “concept”
• stop using symlinked submodules
• reduce reliance on “fancy” String descriptions, and move some of those APIs into the new TerminalColors module
• (superficially) migrate test targets to Testing framework from swift-hash (TODO: actually rewrite the tests to use assertions instead of monkey-patching Result<Void, any Error>)
• update CI so it is no longer 404-ing on unavailable nightly toolchains
• expand CI coverage