Some brittleness parsing basic headers

[mmirate]

When sending a sample of my inbox (selected via an IMAP fetch of "BODY.PEEK[HEADER.FIELDS (MESSAGE-ID DATE SUBJECT FROM)]") through this library, it fails to parse some entries. Exemplars:

"Subject: A message to the campus community\r\nDate: Tue, 27 Nov 2018 16:33:37 +0000\r\nFrom: President@list7.ecc.gatech.edu,\r\n\tG.P.“Bud@list7.ecc.gatech.edu,\r\n\t\" Peterson <president@gatech.edu>\"@list7.ecc.gatech.edu\r\nMessage-id: <git-all.18@anonymous>\r\n\r\n" => Err(TagError("\r\n",))
"Subject: Remembering Kevin Chang and Carter Johns\r\nDate: Thu, 29 Nov 2018 20:17:59 +0000\r\nFrom: John M. Stein <studentlife@studentlife.gatech.edu>\r\nMessage-id: <git-all2.6@anonymous>\r\n\r\n" => Err(MissingHeader("From",))
"Date: Wed, 25 Sep 2019 18:31:08 GMT\r\nMessage-Id: <1569436268867.637983@uptimerobot.com>\r\nFrom: \"Uptime Robot\" <no-reply@uptimerobot.com>\r\nSubject: Introducing Two-Factor Authentication (2FA)\r\n\r\n" => Err(MissingHeader("Date",))
"Subject: Graduate voices in 2021\r\nFrom: \"IB Alumni Network\" <alumni.relations@ibo.org> \r\nDate: Sun, 06 Dec 2020 02:00:43 +1100\r\nMessage-ID: <cm.0200434897081.jutyshk.trkjhrydik.r@cmail20.com>\r\n\r\n" => MissingHeader("From",)

[Mubelotix]

I will investigate these errors. The last 3 seems to be caused by a wrong syntax (actually it's because you cannot construct an Email struct with undefined headers but I will add an easy way to parse incomplete data). The first one is more mysterious. Error handling is hard with this library because there is no way to know where is the error in the email. I tried to add more information in an Error struct but performances dropped significantly. Maybe I will add this behind a feature gate.