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.

contrib/devtools/copyright_header.py

@@ -35,7 +35,6 @@ EXCLUDE_DIRS = [
     "src/leveldb/",
     "src/minisketch",
     "src/secp256k1/",
-    "src/univalue/",
     "src/crc32c/",
 ]