Replace RapidJSON with nlohmann::json

[Robadob]

RapidJSON has become outdated, which has led to increasing maintenance issues.

Static lib builds, but a breaking change is causing 1 test to fail (decision required).

Breaking Changes

• nlohmann json outputs nan/inf as null (this is compatible with how some big js lib works). This means we can't read them back to their original broken value. IOTest.JSON_WriteRead needs amending to account for this, or we need to fudge all float export to actually output it as a string manually.
• nlohmann json outputs empty objects as null, so we're better off ommitting them from our json (e.g. if there are no macro env properties to be logged.