future compat lints

[Mark-Simulacrum]

Once rust-lang/rust#65193 (hopefully) lands, future compatibility lints will have a slightly new registration format:

A @future_incompatible "field" is used in the declare_lint macro, e.g.,

declare_lint! {
    pub DUPLICATE_MACRO_EXPORTS,
    Deny,
    "detects duplicate macro exports"
    @future_incompatible = FutureIncompatibleInfo {
        reference: "issue #35896 <https://github.com/rust-lang/rust/issues/35896>",
        edition: Some(Edition::Edition2018),
    },
}

We will also want to include this comment somehow:

    // Guidelines for creating a future incompatibility lint:
    //
    // - Create a lint defaulting to warn as normal, with ideally the same error
    //   message you would normally give
    // - Add a suitable reference, typically an RFC or tracking issue. Go ahead
    //   and include the full URL, sort items in ascending order of issue numbers.
    // - Later, change lint to error
    // - Eventually, remove lint