refactor: cleanups post unsubtree'ing univalue

Mostly changes to remove src/univalue exceptions from the various linters,
and the required code changes to make them happy. As well as minor doc
changes.

ci/lint/06_script.sh

@@ -18,7 +18,6 @@ export COMMIT_RANGE
 test/lint/git-subtree-check.sh src/crypto/ctaes
 test/lint/git-subtree-check.sh src/secp256k1
 test/lint/git-subtree-check.sh src/minisketch
-test/lint/git-subtree-check.sh src/univalue
 test/lint/git-subtree-check.sh src/leveldb
 test/lint/git-subtree-check.sh src/crc32c
 test/lint/check-doc.py