Currently the json-formatted outputs have no way to unambiguously
determine which kind of message is being output. A consumer can look for
specific fields in the json object (eg "message"), but there's no
guarantee that in future some other kind of output will have a field of
the same name.

This PR adds a `"type"` field to add json outputs which can be used to
unambiguously determine which kind of output it is. The mapping is:

diagnostic: regular compiler diagnostics
artifact: artifact notifications
future_incompat: Report of future incompatibility
unused_extern: Unused crate warnings/errors

This matches the "internally tagged" representation for serde enums.

…sistency.

Avoids an unnecessary intermediate string.

`type` turned out to be controversial.