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/blockchain/mod.rs

@@ -41,6 +41,7 @@ impl Blockchain for OfflineBlockchain {
     }
 }
 
+#[maybe_async]
 pub trait OnlineBlockchain: Blockchain {
     fn get_capabilities(&self) -> HashSet<Capability>;
 
@@ -56,7 +57,7 @@ pub trait OnlineBlockchain: Blockchain {
         database: &mut D,
         progress_update: P,
     ) -> Result<(), Error> {
-        self.setup(stop_gap, database, progress_update)
+        maybe_await!(self.setup(stop_gap, database, progress_update))
     }
 
     fn get_tx(&mut self, txid: &Txid) -> Result<Option<Transaction>, Error>;