[WIP] refactor into const globals

[SimonDanisch]

remove lots of closures and untyped globals, which may or may not be faster and more readable. It at least composes a bit better, to directly call into the underlying libraries for downloads + compression. Last remaining thing that needs to be done is to incorporate the ENV overwrite. I couldn't figure out on first sight how they're supposed to work :D

[musm]

This refactor IMO is a lot more readable than all the globals in the current code.