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.

test/lint/lint-include-guards.py

@@ -22,7 +22,6 @@ EXCLUDE_FILES_WITH_PREFIX = ['src/crypto/ctaes',
                              'src/crc32c',
                              'src/secp256k1',
                              'src/minisketch',
-                             'src/univalue',
                              'src/tinyformat.h',
                              'src/bench/nanobench.h',
                              'src/test/fuzz/FuzzedDataProvider.h']