Add async_poll.rs to lightning-block-sync

In the next commit we'll fetch blocks during initial connection
in parallel, which requires a multi-future poller. Here we add a
symlink to the existing `lightning` `async_poll.rs` file, making it
available in `lightning-block-sync`

Author: TheBlueMatt

lightning-block-sync/src/async_poll.rs

@@ -0,0 +1 @@
+../../lightning/src/util/async_poll.rs

lightning-block-sync/src/lib.rs

@@ -16,9 +16,11 @@
 #![deny(rustdoc::broken_intra_doc_links)]
 #![deny(rustdoc::private_intra_doc_links)]
 #![deny(missing_docs)]
-#![deny(unsafe_code)]
 #![cfg_attr(docsrs, feature(doc_cfg))]
 
+extern crate alloc;
+extern crate core;
+
 #[cfg(any(feature = "rest-client", feature = "rpc-client"))]
 pub mod http;
 
@@ -42,6 +44,9 @@ mod test_utils;
 #[cfg(any(feature = "rest-client", feature = "rpc-client"))]
 mod utils;
 
+#[allow(unused)]
+mod async_poll;
+
 use crate::poll::{ChainTip, Poll, ValidatedBlockHeader};
 
 use bitcoin::block::{Block, Header};