Support official Rust FFI bindings

Currently the most popular mimalloc Rust wrapper is https://github.com/purpleprotocol/mimalloc_rust, but it only exposes mimalloc as a global allocator and all the rich features such as Arenas are missing or only available in the hidden libmimalloc-sys crate. The mimalloc version is also not up-to-date.

There are also other open-source projects building the mimalloc Rust FFI bindings themselves. An example of this is Bun: https://github.com/oven-sh/bun, where it mirrored the mimalloc source code and build a thin FFI layer itself.

It'd be good for the mimalloc to release its official Rust FFI bindings so that all the APIs, documentation, and bug fixes are up to date, just like how ms-quic has a core C implementation and at the same time expose its APIs to C++/C#/Rust: https://github.com/microsoft/msquic

Start by reviewing the existing mimalloc_rust wrapper and its hidden libmimalloc-sys crate, then compare Bun's mimalloc source mirror and FFI layer. Define the scope of the official Rust bindings, including which APIs and documentation must be exposed and how version updates will be maintained. Done means an official, up-to-date binding covering the intended mimalloc APIs.

Written by the indexing model from the issue text.