Add "napi-embedding" feature to allow statically linked N-API symbols

[branchseer]

This PR adds a new feature napi-embedding. When enabled, it loads N-API symbols using extern "C" { ... } declarations instead of libloading.

Why?

I am working on rust-nodejs which embeds Node.js in Rust. The general idea is to build and link Node.js as a static library, and use N-API to communicate between Rust and Node.js.

Neon fits perfectly for the latter part, expect that it tries to load N-API symbols dynamically (#646). This doesn't work in the case of rust-nodejs, because all N-API symbols are statically linked.

[kjvalencik]

@patr0nus Can you explain a little bit more about your use case? While the symbols are being loaded dynamically, they are being loaded from the main process and not from a shared library.

Is there possibly a simpler solution to your problem? For example, if the library that the symbols were loaded from was configurable.

The reason I ask is that I'm hesitant to merge this as is because it will not work on Windows. It also might not work on macOS without additional RUSTC flags.

I am also generally curious about the rust-nodejs project. It seems a like a really neat idea! However, I am a little curious why a user might want to embed Node in Rust instead of Rust in Node.js. Is it about packaging a single binary?

[branchseer]

@kjvalencik

While the symbols are being loaded dynamically, they are being loaded from the main process and not from a shared library.

You are right! It turns out it's because all unused N-API symbols are stripped out by the linker. I'm closing this PR in favor of "-rdynamic" linker flag.

However, I am a little curious why a user might want to embed Node in Rust instead of Rust in Node.js. Is it about packaging a single binary?

Here're my two use cases:

1. Build an Electron-like framework,
2. Allow user script in a Rust application;

Also I think the neon project itself could benefit: with rust-nodejs some tests can run directly in-process, without the need of setting up npm packages. Just an idea though. I'm no expert.

[kjvalencik]

Thanks @patr0nus! It looks like an interesting project. I'll keep an eye on your progress. Cheers!