Expose a Rust stream to JavaScript as a user-downloadable file

[str4d]

The wage::decryptor_for_file API takes a JavaScript File and returns a handle to an age::Decryptor that wraps File.stream(). It would be great if there were a way to similarly expose age::stream::StreamReader as a JavaScript ReadableStream that could be passed back out to the browser, giving it the option to stream-process the decrypted file instead of having the entire decrypted file read into memory.

One issue here is that the Rust handle stores a lifetime, and wasm-bindgen doesn't currently support returning objects with lifetimes (rustwasm/wasm-bindgen#1187). So we may need to hack around that with unsafe, or alternatively have an API that takes a JavaScript TransformStream and writes the decrypted file to it (which would make for a somewhat wonky JavaScript API, but should still enable building JS pipelines).

[str4d]

It turns out that wasm-streams can convert a Box<dyn Stream<Item = Result<JsValue, JsValue>>> into a ReadableStream (source).