[univalue] Lint the univalue code

Summary: Now that we maintain our own version and that we are in sync with core master, it's time to properly format the code with our linter suite.

Test Plan:

ninja all check-univalue
arc lint

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16308