refactor: Replace trait `Decode` with `Decoder`

[jakoschiko]

Replaces the current trait Decode with the trait Decoder in order to improve lifetime issues.

I don't expect this PR to be merged, I just want to demonstrate my ideas. So, what's the problem with the current trait Decode?

pub trait Decode<'a>: Sized {
    fn decode(input: &'a [u8]) -> Result<(&'a [u8], Self), DecodeError>;
}

If I use this trait with type bounds, then I need to choose the lifetime in the wrong place:

fn foo<'a, T: Decode<'a>>() -> T { // The caller chooses a lifetime 'a
    let vec: Vec<u8> = b"foo".to_vec();
    let slice: &[u8] = &vec; // The slice's lifetime does not match lifetime 'a
    T::decode(slice); // Does not compile
}

This prevents me from writing useful abstractions based on Decode. So, here my proposed trait Decoder.

pub trait Decoder {
    type Item<'a>: Sized;

    fn decode<'a>(input: &'a [u8]) -> Result<(&'a [u8], Self::Item<'a>), DecodeError>;
}

It allows me to choose the lifetime at a later point in time:

fn foo<T: Decoder>() {
    let vec: Vec<u8> = b"foo".to_vec();
    let slice: &[u8] = &vec;
    T::decode(slice); // The lifetime is chosen here, now it compiles
}

List of changes:

• I replaced the trait Decode with Decoder
• I introduced the structs CommandDecoder, ResponseDecoder, etc. that implement Decoder
• I replaced the trait DecodeStatic with StaticDecoder that I was able to implement with less code duplication
• I fixed all usages of Decode

If you like the changes, we could do even more:

• For symmetry reasons we replace the trait Encode with Encoder
• We replace CommandDecoder, ResponseDecoder, etc. with CommandCodec, ResponseCodec, etc. that implement both Decoder and Encoder

[coveralls]

Pull Request Test Coverage Report for Build 5797418516

• 29 of 35 (82.86%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.06%) to 93.593%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
imap-codec/src/codec.rs	6	12	50.0%
<!--	Total:	29	35	82.86%	-->

Totals
Change from base Build 5796696502:	-0.06%
Covered Lines:	8706
Relevant Lines:	9302

---

💛 - Coveralls

[duesee]

This makes so much more sense, thanks! :+1: (As discussed offline, let's talk about how, and when, to merge this later.)

[duesee]

As discussed yesterday, I moved the *Decoder structs into codec and renamed them to *Codec. As you proposed, I also removed the StaticDecoder by making it a default trait impl. Is there anything left we want to do in this PR?

Two possible paths to continue: 1) Continue with Encoder or 2) #333.

[jakoschiko]

I suggest this order: merge this, merge #333, add Encoder