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.

Makefile.am

@@ -174,7 +174,6 @@ LCOV_FILTER_PATTERN = \
 	-p "src/leveldb/" \
 	-p "src/crc32c/" \
 	-p "src/bench/" \
-	-p "src/univalue" \
 	-p "src/crypto/ctaes" \
 	-p "src/minisketch" \
 	-p "src/secp256k1" \