Most users of JSONWriter want to fill a string, so instead of having all these
similar implementations, we now have central reusable implementations:
- JSONStringWriteFunc contains a string and writes to it.
- JSONStringRefWriteFunc references a string and writes to it. This is most
useful when the string already exists somewhere, or needs to be returned from
a function (so we avoid another conversion when returning).
Differential Revision: https://phabricator.services.mozilla.com/D154618
