JSON field names in terminal/pager output

[Eric Wong <e@80x24.org>]

Are "f", "s", "t", "c" acceptable field names to show in JSON
output?  (instead of from/subject/to/cc)

The single-chars have been used in the search queries for as
long as we've had search.  And I stole that UI bit from
mairix(1), so there's prior art.

Anyways, the current JSON output looks something like the
following, comments inline

[{
  "blob": "d21717dae7e18dbc9efcd5ca57fe2ee92747bc06",
  "docid": 41946,
  "dt": "2021-01-05T09:24:49Z",

  # dt: is not an exact match for current WWW behavior which
  # needs YYYYMMDDHHMMSS (all digits).  Getting Xapian to parse
  # dates from Perl (w/o custom C++) isn't possible, yet.
  # dt: is the date header, "UTCDate" in JMAP.

  "f": "Eric Wong <e@80x24.org>",

  # "from": might be more obvious, but seeing it thousands of
  # times every message might be too much

  "m": "<20210105092449.GA22853@dcvr>",

  # mid/msgid/message-id

  "rcvd": "2021-01-05T09:24:49Z",
  # IMAP calls this INTERNALDATE, JMAP calls it "receivedAt"

  "references": [
    "<20210105090437.22801-1-e@80x24.org>",
    "<20210105090437.22801-5-e@80x24.org>"
  ],

  # we don't actually support searching on rereference, yet;
  # not sure if it's needed since we already do thread # expansion

  "relevance": 13,

  # relevance is ->get_percent from Xapian

  "s": "JSON pretty-printing [was: [4/4] ... (and maybe lei)]",
  "t": "meta@public-inbox.org"

  # Subject and To: headers
}, {
  # Another message, we do "}, {" to save vertical white space
  rather than "},\n{"

  "blob": "0ee1d7d9ec9b29c1d8f103033ed06e9e48e6ebfe",
  "docid": 41930,

  # side note: it's probably not worth displaying docid for
  # ephemeral search indices like LeiXSearch

  "dt": "2021-01-05T09:04:36Z",
  "f": "Eric Wong <e@80x24.org>",
  "m": "<20210105090437.22801-4-e@80x24.org>",
  "rcvd": "2021-01-05T09:04:38Z",
  "references": [
    "<20210105090437.22801-1-e@80x24.org>"
  ],
  "relevance": 36,
  "s": "[PATCH 3/4] lei: use client env as-is, drop daemon-env command",
  "t": "meta@public-inbox.org"
}, {


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