Compilation fails for wasm-bindgen and async-std

[wackazong]

I use the crate with wasm-bindgen and async-std and it fails to compile with the following errors.

error: future cannot be sent between threads safely
   --> /Users/ander/.cargo/registry/src/github.com-1ecc6299db9ec823/backoff-0.4.0/src/future.rs:237:9
    |
237 |         Box::pin(::async_std_1::task::sleep(dur))
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `std::marker::Send` is not implemented for `*mut u8`
note: future is not `Send` as this value is used across an await
   --> /Users/ander/.cargo/registry/src/github.com-1ecc6299db9ec823/async-std-1.12.0/src/io/timeout.rs:43:5
    |
40  |         timeout: timer_after(dur),
    |                  ---------------- has type `async_std::utils::timer::Timer` which is not `Send`
...
43  |     .await
    |     ^^^^^^ await occurs here, with `timer_after(dur)` maybe used later
44  | }
    | - `timer_after(dur)` is later dropped here
    = note: required for the cast from `impl Future<Output = ()>` to the object type `dyn Future<Output = ()> + std::marker::Send`

error: future cannot be sent between threads safely
   --> /Users/ander/.cargo/registry/src/github.com-1ecc6299db9ec823/backoff-0.4.0/src/future.rs:237:9
    |
237 |         Box::pin(::async_std_1::task::sleep(dur))
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future is not `Send`
    |
    = help: the trait `std::marker::Send` is not implemented for `(dyn FnMut() + 'static)`
note: future is not `Send` as this value is used across an await
   --> /Users/ander/.cargo/registry/src/github.com-1ecc6299db9ec823/async-std-1.12.0/src/io/timeout.rs:43:5
    |
40  |         timeout: timer_after(dur),
    |                  ---------------- has type `async_std::utils::timer::Timer` which is not `Send`
...
43  |     .await
    |     ^^^^^^ await occurs here, with `timer_after(dur)` maybe used later
44  | }
    | - `timer_after(dur)` is later dropped here
    = note: required for the cast from `impl Future<Output = ()>` to the object type `dyn Future<Output = ()> + std::marker::Send`