feat!: Add type-level traversal skip attributes

[tisonkun]

Summary

• Add type-level #[traverse(skip_self)] and #[traverse(skip_children)] attributes.
• Keep field and variant #[traverse(skip)] scoped to skipping those children.
• Document the attribute split and add derive behavior tests for immutable and mutable traversal.

Fixes #17.

Tests

• cargo fmt --check
• cargo test --workspace --all-features
• cargo clippy --workspace --all-features --all-targets -- -D warnings

[Copilot]

Pull request overview

This PR introduces a clearer split between type-level traversal control (skip visiting the type itself vs. skipping traversal of its children) and child-level skipping on fields/variants, updating documentation and adding derive-behavior tests to validate the new semantics.

Changes:

• Add type-level #[traverse(skip_self)] and #[traverse(skip_children)] derive attributes and update derive generation accordingly.
• Document the attribute split in crate docs and README.
• Add integration tests covering default traversal and the new type-level skip behaviors for both immutable and mutable traversal.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
xtask/src/main.rs	Updates lint helpers to run fmt/clippy via +nightly.
traversable/tests/test_derive_attributes.rs	Adds tests validating new type-level traversal attribute behavior.
traversable/src/lib.rs	Updates public documentation to describe the new attribute split.
traversable-derive/src/lib.rs	Implements parsing and codegen for skip_self / skip_children at the type level.
README.md	Documents the new type-level attributes alongside field/variant skip.
.github/workflows/ci.yml	Updates CI runner versions, checkout action version, and toolchain/lint steps.

Comments suppressed due to low confidence (1)

.github/workflows/ci.yml:68

• In the test matrix job, Swatinem/rust-cache runs before the selected toolchain is installed. For consistent cache keys across the matrix (and to avoid restoring artifacts built with a different toolchain), it’s better to install the toolchain first and then run Swatinem/rust-cache.

      - uses: actions/checkout@v6
      - uses: Swatinem/rust-cache@v2
      - name: Delete rust-toolchain.toml
        run: rm rust-toolchain.toml
      - name: Install toolchain
        uses: dtolnay/rust-toolchain@master
        with:
          toolchain: ${{ matrix.rust-version }}

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