Organize beacon_chain typing

- Implements ClientTypes
- New network BeaconChain type for the networking service

beacon_node/network/src/beacon_chain.rs

@@ -0,0 +1,27 @@
+use beacon_chain::BeaconChain as RawBeaconChain;
+use beacon_chain::{
+    db::ClientDB, fork_choice::ForkChoice, parking_lot::RwLockReadGuard, slot_clock::SlotClock,
+    CheckPoint,
+};
+
+/// The network's API to the beacon chain.
+pub trait BeaconChain: Send + Sync {
+    fn head(&self) -> RwLockReadGuard<CheckPoint>;
+
+    fn finalized_head(&self) -> RwLockReadGuard<CheckPoint>;
+}
+
+impl<T, U, F> BeaconChain for RawBeaconChain<T, U, F>
+where
+    T: ClientDB + Sized,
+    U: SlotClock,
+    F: ForkChoice,
+{
+    fn head(&self) -> RwLockReadGuard<CheckPoint> {
+        self.head()
+    }
+
+    fn finalized_head(&self) -> RwLockReadGuard<CheckPoint> {
+        self.finalized_head()
+    }
+}