✨ QRESYNC: Add vanished keyword to #uid_fetch

[nevans]

The vanished kwarg to #uid_fetch can be used to request a list all of the message UIDs in set that have been expunged since changedsince. Setting vanished to true prepends a VanishedData object to the returned array. If the server does not return a VANISHED response, an empty VanishedData object will still be added.

For example:

  imap.enable("QRESYNC") # must enable before selecting the mailbox
  imap.select("INBOX")
  # the first value in the returned array is a VanishedData object
  vanished, *fetched = imap.uid_fetch(301..500, %w[FLAGS],
                                      changedsince: 12345,
                                      vanished: true)

The QRESYNC capabability must be enabled. RFC7162

Alternatives for return type

This PR chose to prepend a VanishedData object to the returned array. Because it only does this when the vanished: true kwarg is sent, it should be fully backward compatible. Other API choices were considered:

• Make no changes and continue returning only an array of fetch data. This is not very user friendly.
• Return a tuple of [vanished, fetched_array]. I strongly prefer a flat array of responses ([vanished, *fetched_array]). IMO, it's more consistent with the original API, and just as easy to work with.
• Return a new FetchResult type, with a #vanished method.
  • This could subclass Array, like SearchResult. That was done to minimize any backward compatibility issues. Also, there's no simple way to distinguish between a modseq integer and a UID or sequence number. But I'd prefer not to subclass Array.
  • This could delegate to the fetch data array. This is my preferred approach. But I don't want to only enable it for vanished. I'd prefer to always return FetchResult according to a fetch_result config option. And that wouldn't be backward compatible enough to enable by default for the next release.

[nevans]

@shugo Before I merge this, do you have any thoughts on the vanished, *fetched = uid_fetch(...) API design choice?