Implementing unwind safety

Closure invoked after being dropped in `Drop` impls of `IntoStream` and `IntoAsyncRead`

Missing docs how to run the fetch_as_stream.rs example

Readable stream from chunked transfer encoding seems to miss some chunks

calling into_async_read panics whereas into_stream works well

Emscripten shared library support

Cannot get JS -> Rust -> JS `WritableStream` to work on all browsers