Add the unstable feature to async-std to fix compilation on windows

Cargo.toml

@@ -20,7 +20,7 @@ contents are never required to be entirely resident in memory all at once.
 """
 
 [dependencies]
-async-std = "1.6.0"
+async-std = { version = "1.6.0", features = ["unstable"] }
 filetime = "0.2.8"
 pin-project = "1.0.8"