Add a crossbeam sharded rwlock backend

[gz]

Use crossbeam-utils::sync::ShardedLock behind a new crossbeam feature when parking_lot is off.

Keep the existing wrapper API, document the feature, and fail fast when both lock backends are enabled.

Discussion in #108

[Copilot]

Pull request overview

Adds an optional crossbeam feature to use crossbeam_utils::sync::ShardedLock as the crate’s internal RwLock implementation when parking_lot is disabled.

Changes:

• Introduce crossbeam Cargo feature and optional crossbeam-utils dependency.
• Switch RwLock backend to ShardedLock under cfg(all(not(feature="parking_lot"), feature="crossbeam")).
• Update crate docs and add a compile_error! guard for mutually exclusive parking_lot/crossbeam features.

Reviewed changes

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

File	Description
src/rw_lock.rs	Adds crossbeam-based RwLock type aliases and adjusts RwLock::new const-ness under crossbeam.
src/lib.rs	Documents the new backend selection and adds a mutual-exclusion compile-time check.
Cargo.toml	Defines the crossbeam feature and adds crossbeam-utils as an optional dependency.

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[Copilot]

Pull request overview

This PR adds an alternative RwLock backend based on crossbeam_utils::sync::ShardedLock, selectable via a new crossbeam Cargo feature when parking_lot is disabled, while keeping the existing wrapper API and documenting the feature behavior.

Changes:

• Add a crossbeam feature and optional crossbeam-utils dependency to support ShardedLock as the RwLock backend.
• Document the new feature and add a compile-time error when parking_lot and crossbeam are both enabled.
• Update CI to compile/test the new crossbeam configuration and avoid invalid --all-features runs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/rw_lock.rs	Adds ShardedLock as an alternate RwLock implementation behind the crossbeam feature.
src/lib.rs	Documents the new backend selection and enforces mutual exclusivity via compile_error!.
Cargo.toml	Introduces the crossbeam feature and optional crossbeam-utils dependency.
.github/workflows/ci.yml	Adjusts CI commands to cover the new feature without using --all-features.

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