move extractors and generic schema utilities out of handler.rs

[davepacheco]

I felt like handler.rs was getting a bit big and full of unrelated stuff. Here I'm separating the extractors into a submodule with the eventual intent on putting different ones into different files within extractors. This involved moving some functions used by both handler.rs and the new extractor module into schema_util.rs (analogous to http_util.rs).

I believe nothing has changed in this PR except for moving code from one file to another and adjusting imports / "mod" statements.

This depends on #553 -- will update the base branch once that one lands and I'll keep this in "draft" for the meantime.

[davepacheco]

Sorry for the force push and general noise.

[ahl]

I really like this reorganization. I said "oooh" aloud when I saw that extractor was its own mod--like it a lot.

api_description.rs has a bunch of j2oas_ prefixed functions; would those make sense to move into schema_util.rs? No or Not now are fine answers.

[davepacheco]

Thanks for the review.

api_description.rs has a bunch of j2oas_ prefixed functions; would those make sense to move into schema_util.rs? No or Not now are fine answers.

That looks like a good idea. I will do it in a follow-up PR.