Wasm support

[Luthaf]

Is there any possibility of wasm32-unknown-unknown support at any point down the line?

Originally posted by @Yoshanuikabundi in #22 (comment)

[Luthaf]

Moving the discussion to a separate issue since this question is quite complex =)

---

For context, chemfiles is written in C++, and exposes a C API. This repository contains the Rust FFI binding to the C API.

The C++ code can be compiled to WASM using emscripten, this is the basis for the (unreleased) JS bindings to chemfiles in https://github.com/chemfiles/chemfiles.js.

Unfortunately, there are some difference between the emscripten WASM target and the Rust wasm32-unknown-unknown target, so I don't know if we can use the resulting WASM code together with Rust.

The alternative would be to compile the C++ code without emscripten, maybe using LLVM facilities directly trying to make it as compatible with rustc wasm32-unknown-unknown target as possible. The issue here is that emscripten provides a lot of functionalities (mainly a virtual filesystem giving access to pseudo-files in a browser context), which would have to be replicated with the new compiler

Overall, this is something I would like to enable, but will require a lot of work & knowledge about both WASM & FFI.