web-sys: arrayBuffer function on Blob API

[anlumo]

Motivation

Blob has an arrayBuffer function documented here. It's very convenient to use from Rust due to using Promises that can easily be converted to Futures, but it's not exposed in web-sys.

Proposed Solution

While it's not supported on all browsers, it would be great to have this function exposed by the crate.

Alternatives

Currently I'm using Reflect to get the function and call it directly, but this is a lot of code and probably also not as fast.

Alternatively, there's a callback-based API exposed as readAsArrayBuffer, but that's very cumbersome to use due to Rust's memory model.