Drastically simplify changes to verify.go

[aarongable]

This removes a whole layer of abstraction from verify.go, making the changes easier to reason about. It also undoes some of the unnecessary comment updates, making the diff easier to review. I think that, with this change incorporated, we will be very likely to accept letsencrypt#8796.

[CLAassistant]

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[aarongable]

Hmm, signing the CLA doesn't seem like the right move here -- this is a contribution not to a ToB repo, but to a ToB PR against my own repo. I could bypass the ToB PR and make my own PR against my own repo, but that feels rude. Let me know what the right next step is.