[unvalue] Run clang-tidy against the univalue codebase

Summary: And fix comments and style nits as we go.

Test Plan:
With Clang Tidy:

ninja check check-univalue

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16310