Add support for custom allocators in `Vec` This follows the [roadmap](https://github.com/rust-lang/wg-allocators/issues/7) of the allocator WG to add custom allocators to collections. r? `@Amanieu` This pull request requires a crater run. ### Prior work: - #71873: Crater-test to solve rust-lang/wg-allocators#1 - [`alloc-wg`](https://github.com/TimDiekmann/alloc-wg)-crate