Squashed 'lib/json/' changes from 80ee324..f52b5a7

f52b5a7 Add at method to object + fix const correctness of has_member

git-subtree-dir: lib/json
git-subtree-split: f52b5a78721156be21d18a23c7dc7976167444c1
Merge commit '466f251101f0b7153af75f69c4cfc876382bd513' update json library