refactor: use `Fragmentizer` from `imap-codec`

[coveralls]

Pull Request Test Coverage Report for Build 9914379481

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 81.583%

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Totals
Change from base Build 9914368636:	0.2%
Covered Lines:	1010
Relevant Lines:	1238

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💛 - Coveralls

[jakoschiko]

Closes #238