Look-up functions for ReadOnlyNetworkGraph

ReadOnlyNetworkGraph uses BTreeMap to store its nodes and channels, but
these data structures are not supported by C bindings. Expose look-up
functions on these maps in lieu of such support.

lightning/src/routing/gossip.rs

@@ -1602,13 +1602,23 @@ impl ReadOnlyNetworkGraph<'_> {
 		&*self.channels
 	}
 
+	/// Returns information on a channel with the given id.
+	pub fn get_channel(&self, short_channel_id: u64) -> Option<&ChannelInfo> {
+		self.channels.get(&short_channel_id)
+	}
+
 	/// Returns all known nodes' public keys along with announced node info.
 	///
 	/// (C-not exported) because we have no mapping for `BTreeMap`s
 	pub fn nodes(&self) -> &BTreeMap<NodeId, NodeInfo> {
 		&*self.nodes
 	}
 
+	/// Returns information on a node with the given id.
+	pub fn get_node(&self, node_id: &NodeId) -> Option<&NodeInfo> {
+		self.nodes.get(node_id)
+	}
+
 	/// Get network addresses by node id.
 	/// Returns None if the requested node is completely unknown,
 	/// or if node announcement for the node was never received.