Squashed 'json/' changes from 0ba6722..ddef9b0

ddef9b0 Add a note to the README about running sanity checks.
f93cc55 Shorten the descriptions slightly.
dd9a876 Merge remote-tracking branch 'fge/develop' into develop
bc84af3 Add string length tests for supplementary Unicode code points

git-subtree-dir: json
git-subtree-split: ddef9b0415461ca948f3c0605e2eb2cc40981b95
Skip the new unicode tests on narrow builds which don't know about surrogates.