`slice-dst`: Support allocator API

It seems that `slice-dst` can only be allocated with the global allocator. Although allocator_api is currently unstable, there is a widely used crate [allocator-api2](https://docs.rs/allocator-api2) that mirrors types and traits from [allocator_api](https://doc.rust-lang.org/unstable-book/library-features/allocator-api.html). Therefore, I think `slice-dst` can introduce a feature that enables allocator API mirrored by `allocator-api2` similar to [bumpalo](https://docs.rs/bumpalo).

Start by reading the slice-dst implementation and comparing bumpalo's allocator-api2 support. Review the allocator_api and allocator-api2 traits and types to determine the feature's integration points. Done means slice-dst can use allocator-api2-backed allocation without requiring the global allocator, with coverage for the supported allocator paths.

Written by the indexing model from the issue text.