Expose nodes and channels in C bindings

ReadOnlyNetworkGraph uses BTreeMap to store its nodes and channels, but
these data structures are not supported by C bindings. Expose them as
Vecs instead.

lightning/src/routing/gossip.rs

@@ -1602,13 +1602,25 @@ impl ReadOnlyNetworkGraph<'_> {
 		&*self.channels
 	}
 
+	#[cfg(c_bindings)]
+	/// Returns all known valid channels' short ids along with announced channel info.
+	pub fn channels_vec(&self) -> Vec<(u64, ChannelInfo)> {
+		self.channels.iter().map(|(id, info)| (*id, info.clone())).collect()
+	}
+
 	/// 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
 	}
 
+	#[cfg(c_bindings)]
+	/// Returns all known nodes' public keys along with announced node info.
+	pub fn nodes_vec(&'_ self) -> Vec<(NodeId, NodeInfo)> {
+		self.nodes.iter().map(|(id, info)| (*id, info.clone())).collect()
+	}
+
 	/// 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.