feat: support intrusive_adapter! access nested field

[MrCroxx]

As the nested field access feature with offset_of is supported since Rust stable 1.82.0, it is time to support nested field access with intrusive_adapter! macro too!

Changes:

1. Support nested field access with macro intrusive_adapter! and container_of.
2. Replace : with => to identify the link for the intrusive data structure. To support nested field access with offset_of, it requires the path to the link to be identified by $($fields:expr)+. Only =>, ,, ; can be used with the tailing expr. And => suits the scenario here.

Example: (copied from the test)

#[cfg(test)]
mod tests {
    use crate::LinkedListLink;
    use std::rc::Rc;

    struct Obj {
        link: LinkedListLink,
    }

    struct Wrapper {
        obj: Obj,
    }

    intrusive_adapter! {
        /// Test doc comment
        ObjAdapter1 = Rc<Obj>: Obj { link => LinkedListLink }
    }

    intrusive_adapter! {
        /// Test doc comment
        WrapperAdapter1 = Rc<Wrapper>: Wrapper { obj.link => LinkedListLink }
    }
}

Requirements:

1. Bump MSRV to 1.82.0 .
2. Bump the major version of intrusive-collections.

FYI:

1. Rust 1.82.0 release log: https://releases.rs/docs/1.82.0/#language
2. offset_of_nested stabilization PR: Stabilize offset_of_nested rust-lang/rust#128284