Unable to use `async_std::stream::interval` on `wasm32-unknown-unknown` target

In `async-std@1.6.0-beta.1`, [`async_std::stream::interval`](https://docs.rs/async-std/1.6.0-beta.1/async_std/stream/fn.interval.html) is feature gated by the `"unstable"` feature. Enabling that feature [brings in `smol`](https://github.com/async-rs/async-std/blob/v1.6.0-beta.1/Cargo.toml#L45), which depends on `socket2`, which at this time cannot be compiled for the `wasm32-unknown-unknown` target (https://github.com/alexcrichton/socket2-rs/issues/35).

I'm not sure that there's enough people to maintain separate features per item in `async-std`, so if we can't stabilize `stream::interval`, then this is simply an informative issue while `smol`'s WASM support is being built (https://github.com/stjepang/smol/issues/87).