feat: restore may_dangle drop support

[Chandana5599]

Summary

• Restore may_dangle support behind an opt-in feature flag.
• Preserve the existing Drop implementation when the feature is disabled.
• Add a regression test for the example in issue re-implement may_dangle drop on SmallVec #756.

Testing

• cargo test --features allocator-api2
• cargo +nightly test --features "allocator-api2 may_dangle"
• cargo +nightly fmt --check
• git diff --check

Fixes #756

[Chandana5599]

Hi maintainers, this PR restores may_dangle drop support behind an opt-in feature flag and includes a regression test for #756. The workflow is currently awaiting approval, and the PR requires a review. Could someone please approve the workflow and review the changes when convenient? Thank you!

[alejandro-vaz]

this can indeed be made simpler if we had a prepend function for the unsafe, then we would just have cfg_attrs and it'd be much simpler

give me some time to think about it

[alejandro-vaz]

alright with #764 this should be much easier to implement

it is simply a conditional may_dangle attribute behind cfg_attr + a prepend of unsafe

everything else stays the same and we cut 50 lines of code