Make CodeMap lock-free

[Robbepop]

Since #1122 Wasmi's internal CodeMap, that stores all the data of functions stored in the Engine, is put entirely behind a Mutex based lock for all accesses.

Link to code_map.rs: https://github.com/wasmi-labs/wasmi/blob/main/crates/wasmi/src/engine/code_map.rs

Before #1122, it was put behind an RwLock and was read-locked whenever Wasmi exeucted a function which was the root cause for the dead-lock that #1122 has fixed.

The downsides of #1122 is that the performance of call-intense workload have regressed by roughly 5-10% which was acceptable given the importance of fixing the dead-lock.

However, ideally we want a lock-free version of CodeMap with hopefully improved performance, especially for accessing pre-compiled Wasm functions. Access of pre-compiled Wasm functions is part of the hot-path of the Wasmi executor and thus its performance is critical.

An inspiring blog post about a lock-free append-only Vec implementation in Rust:
https://ibraheem.ca/posts/a-lock-free-vector/

This issue is about tracking progress for the lock-free CodeMap implementation.