MessageValue and friends are pure value objects and newable, so
it makes sense for them to be (de)serializable too. There are some
places where we want to serialize messages, such as in ParserOutput.
The structure of the resulting JSON is inspired by the way we
represent Message objects as plain values elsewhere in MediaWiki,
e.g. StatusValue::getStatusArray().
Co-Authored-By: C. Scott Ananian <cscott@cscott.net>
Depends-On: Ia32f95a6bdf342262b4ef044140527f0676402b9
Depends-On: I7bafe80cd36c2558517f474871148286350a4e76
Change-Id: Id47d58b5e26707fa0e0dbdd37418c0d54c8dd503
c7f52f0ddb