refactor(types): borrow structure from imap-types

[soywod]

This PR explodes the actual lib.rs into submodules, similar to imap-types.

[duesee]

I very much like the idea but have some concerns. I tried to find/use a tool that would allow me to verify that the code was mostly only moved, such as https://github.com/GumTreeDiff/gumtree. There was some tool able to do it, but I could not find it again. And since I can't realistically review this PR, I have to ask... how did you separate the code into files?

Also... for a refactor... +3358 and -981 lines looks... well... off?

[soywod]

See #22 (comment) for more context. The code has been separated pretty much the same as imap-types, I will comment each files. The diff is less scary as it looks. While it has been generated, I checked files one by one and it seems legit (at least as a good base to iterate on).

[duesee]

I think I should better add another comment before we waste time. I don't feel well taking AI-generated code currently. It feels like too much too quickly, and since I can only do small reviews, I would not feel well to provide this code for other people to use. And yet... I would love to push smtp-codec over the finish line. Also needed it a few times myself.

Good thing is: SMTP is much simpler than IMAP. I think we can get pretty far quickly w/o relying so much on AI.

Shall we talk this through, maybe next week? :-)

[soywod]

Sure, let's do this!