Make the blockchain interface async again on wasm32-unknown-unknown

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.

src/lib.rs

@@ -5,6 +5,12 @@ extern crate serde;
 #[macro_use]
 extern crate serde_json;
 
+#[cfg(target_arch = "wasm32")]
+#[macro_use]
+extern crate async_trait;
+#[macro_use]
+extern crate magical_macros;
+
 #[cfg(test)]
 #[macro_use]
 extern crate lazy_static;