Various improvements to target handling

[calebzulawski]

• Rely on rustc for implied feature handling
• Remove CPU multiversion targets due to instability between rustc versions
• Expand and correct simd targets (fixes simd enables avx2 without bmi2 #51)
• Make target-features dependency optional

[Copilot]

Pull request overview

Improves target parsing, SIMD coverage, feature gating, and no_std support.

Changes:

• Delegates feature validation to rustc and removes CPU specifications.
• Expands SIMD targets with version/nightly gating.
• Makes target-features optional and strengthens CI coverage.

Reviewed changes

Copilot reviewed 14 out of 15 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
README.md	Updates MSRV badge.
multiversion/tests/simd.rs	Adds nightly architecture gates.
multiversion/tests/helpers.rs	Gates optional dependency tests.
multiversion/tests/cfg.rs	Updates target configuration tests.
multiversion/src/lib.rs	Adds no_std and feature gating.
multiversion/Cargo.toml	Defines MSRV and optional features.
multiversion-macros/src/target.rs	Simplifies target parsing.
multiversion-macros/src/simd.rs	Defines expanded SIMD targets.
multiversion-macros/src/multiversion.rs	Uses centralized SIMD targets.
multiversion-macros/src/lib.rs	Registers SIMD and target macros.
multiversion-macros/src/dispatcher.rs	Generates deferred selected-target macros.
multiversion-macros/Cargo.toml	Updates features and dependencies.
CHANGELOG.md	Documents user-facing changes.
Cargo.lock	Updates dependency resolution.
.github/workflows/ci.yml	Expands platform and feature coverage.

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[Shnatsel]

Is there any timeline for when this is going to ship to crates.io?

I'd like to be able to point people to the #[multiversion(targets = "simd")] mechanism from my "state of SIMD" blog post this year.

[calebzulawski]

I'll see if I can get a release this weekend!