feat: Implement RFC 2342 (NAMESPACE) in imap-codec

[qbit-0]

No description provided.

[duesee]

Question:

Since the ABNF seems to be ...

Namespace = nil / "(" 1*Namespace-Descr ")"

... would it make sense to just use a Vec<NamespaceDescription> without the Option and make it so that an empty vector is encoded as nil?

Currently, None would mean nil but someone could still instantiate Some(vec![]) which... well... should also mean nil? Unclear.

For cases where we want to encode a vector with at least one element, we have Vec1. But instead using Option<Vec1>, Vec is even simpler.

[qbit-0]

qbit-0@e9a90bc

[duesee]

With some minor corrections, this looks already good! See b5a3691.

$ cargo run --example=server --features=ext_namespace
# Parsing of IMAP commands

"C:" denotes the client,
"S:" denotes the server, and
".." denotes the continuation of an (incomplete) command, e.g., due to the use of an IMAP literal.

Note: "\n" will be automatically replaced by "\r\n".

--------------------------------------------------------------------------------------------------

Enter IMAP commands (or "exit").

S: * OK ...
C: a namespace
Command {
    tag: Tag("a"),
    body: Namespace,
}
C: 

$ cargo run --example=client --features=ext_namespace
# Parsing of IMAP greeting and responses

"S:" denotes the server.
".." denotes the continuation of an (incomplete) response, e.g., due to the use of an IMAP literal.

Note: "\n" will be automatically replaced by "\r\n".

--------------------------------------------------------------------------------------------------

Enter intial IMAP greeting followed by IMAP responses (or "exit").

S: * OK ...
Greeting {
    kind: Ok,
    code: None,
    text: Text("..."),
}
S: * NAMESPACE (("INBOX." ".")) NIL NIL 
Data(
    Namespace {
        personal: [
            Namespace {
                prefix: String(
                    Quoted(
                        Quoted("INBOX."),
                    ),
                ),
                delimiter: Some(
                    QuotedChar(
                        '.',
                    ),
                ),
                extensions: [],
            },
        ],
        other: [],
        shared: [],
    },
)
S: * NAMESPACE (("" "/")) (("Other Users/" "/")) NIL
Data(
    Namespace {
        personal: [
            Namespace {
                prefix: String(
                    Quoted(
                        Quoted(""),
                    ),
                ),
                delimiter: Some(
                    QuotedChar(
                        '/',
                    ),
                ),
                extensions: [],
            },
        ],
        other: [
            Namespace {
                prefix: String(
                    Quoted(
                        Quoted("Other Users/"),
                    ),
                ),
                delimiter: Some(
                    QuotedChar(
                        '/',
                    ),
                ),
                extensions: [],
            },
        ],
        shared: [],
    },
)
S: 

[duesee]

Could you run cargo +nightly fmt? I feel that this PR is almost ready to review!