Module refactors

[camsteffen]

A set of refactors, mostly around modules.rs to make code DRYer and simpler. Review one commit at a time and note that some commits are prep work for later commits.

[ytmimi]

I'd like to land #5611 before taking a look at these changes. I'm not 100% sure if that will impact what you're trying to do here but I wanted to give you a heads up

[camsteffen]

I removed some commits to keep this PR focused on a series of related changes. I also took things a bit further by introducing an arena. I think this ultimately makes things simpler overall. I also added explanations on most individual commit messages.

(I know we're waiting on #5611. It looks like that will be a pretty simple git conflict.)