- Also upgrade reqwest
- Switch to `tokio::runtime::Builder::new_single_thread()` because
`tokio::runtime::Runtime::new()` changed it's behavior to create a
multithreaded runtime.
- `enable_all` enables time and io resource drivers as explained
[here](https://docs.rs/tokio/0.2.24/tokio/runtime/index.html#resource-drivers)
The procedural macro `#[maybe_async]` makes a method or every method of a trait
"async" whenever the target_arch is `wasm32`, and leaves them untouched on
every other platform.
The macro `maybe_await!($e:expr)` can be used to call `maybe_async` methods on
multi-platform code: it expands to `$e` on non-wasm32 platforms and to
`$e.await` on wasm32.
The macro `await_or_block!($e:expr)` can be used to contain async code as much
as possible: it expands to `tokio::runtime::Runtime::new().unwrap().block_on($e)`
on non-wasm32 platforms, and to `$e.await` on wasm32.
