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/wallet/mod.rs

@@ -709,6 +709,7 @@ where
     B: OnlineBlockchain,
     D: BatchDatabase,
 {
+    #[maybe_async]
     pub fn new(
         descriptor: &str,
         change_descriptor: Option<&str>,
@@ -738,7 +739,7 @@ where
             None => None,
         };
 
-        let current_height = Some(client.get_height()? as u32);
+        let current_height = Some(maybe_await!(client.get_height())? as u32);
 
         Ok(Wallet {
             descriptor,
@@ -752,6 +753,7 @@ where
         })
     }
 
+    #[maybe_async]
     pub fn sync(
         &self,
         max_address: Option<u32>,
@@ -811,15 +813,16 @@ where
             self.database.borrow_mut().commit_batch(address_batch)?;
         }
 
-        self.client.borrow_mut().sync(
+        maybe_await!(self.client.borrow_mut().sync(
             None,
             self.database.borrow_mut().deref_mut(),
             noop_progress(),
-        )
+        ))
     }
 
+    #[maybe_async]
     pub fn broadcast(&self, tx: Transaction) -> Result<Txid, Error> {
-        self.client.borrow_mut().broadcast(&tx)?;
+        maybe_await!(self.client.borrow_mut().broadcast(&tx))?;
 
         Ok(tx.txid())
     }