feat: impl Spawn for Core

so that sub systems are able to spawn tasks

examples/test/.pijulignore

@@ -1 +0,0 @@
-/data

examples/test/src/main.rs

@@ -2,11 +2,24 @@ use ::anyhow::Result;
 use ::crdt_enc_gpgme::KeyHandler;
 use ::crdt_enc_sodium::EncHandler;
 use ::crdt_enc_tokio::Storage;
+use ::futures::task;
 use ::uuid::Uuid;
 
 const CURRENT_DATA_VERSION: Uuid = Uuid::from_u128(0xaadfd5a6_6e19_4b24_a802_4fa27c72f20c);
 
 const SUPPORTED_DATA_VERSIONS: &[Uuid] = &[CURRENT_DATA_VERSION];
+#[derive(Debug)]
+struct TokioSpawn;
+
+impl crdt_enc::CoreSpawn for TokioSpawn {}
+
+impl task::Spawn for TokioSpawn {
+    fn spawn_obj(&self, future: task::FutureObj<'static, ()>) -> Result<(), task::SpawnError> {
+        // drop tokios `JoinHandle`
+        tokio::spawn(future);
+        Ok(())
+    }
+}
 
 #[tokio::main(flavor = "current_thread")]
 async fn main() -> Result<()> {
@@ -18,6 +31,7 @@ async fn main() -> Result<()> {
     let cryptor = EncHandler::new();
     let key_cryptor = KeyHandler::new();
     let open_options = crdt_enc::OpenOptions {
+        spawn: Box::new(TokioSpawn),
         storage,
         cryptor,
         key_cryptor,