remove spaghetti, or: restructure almost all the bare functions into struct members

[klardotsh]

Look, was this strictly necessary? probably not: technically things built as of #5. however, I had a litany of TODO comments in the source alluding to future refactors, and I finally got tired of seeing them, and also, in developing the CI commits in #5 I had to allow Clippy warnings in CI because of vestigial terrible decisions from the "hack and go" stage of development. let's get rid of those artifacts and allow Clippy to scream as loud as it so desires, eh? Singing is truly the way of life, after all.

• Commit 1 is all regarding bundling user_script_registry_key with the actual Lua VM that generated it, rather than passing each as bare arguments all over the stack
• Commit 2 is... everything else? I never hit a commitable intermediate state while going through the hundreds of type errors during this refactor, sorry. Basically, the step_* files now became non-step files that speak with Pipeline (a new struct) over strict interfaces, rather than the spaghetti I'd previously allowed. Thus, all StepHandlers now implement StepHandler and are semi-plugins to Pipeline ("semi" because they're still plumbed one-by-one and not loaded at runtime, but at least, they have extremely limited interfaces to speak over now).
• Commit 3 is cranking up CI anger, since the warnings it previously complained about are now fixed, and when presented a linter, I can't help but put it on the strictest settings possible. Clippy warnings will now fail the build, as they should.

screenshot proof that UX has not regressed (plus, we have cargo test in CI with #5, which tests some units):

[pdpol]

Awesome restructuring!