Implement skeleton network/sync framework.

beacon_node/sync/src/lib.rs

@@ -3,8 +3,9 @@
 /// Stores the various syncing methods for the beacon chain.
 mod simple_sync;
 
-pub use crate::SimpleSync;
+pub use simple_sync::SimpleSync;
 
+/// Currently implemented sync methods.
 pub enum SyncMethod {
     SimpleSync,
 }