Move Channel fields into ChannelContext struct

This is a first step for simplifying the channel state and introducing
new unfunded channel types that hold similar state before being promoted
to funded channels.

Essentially, we want the outer `Channel` type (and upcoming channel types)
to wrap the context so we can apply typestate patterns to the that wrapper
while also deduplicating code for common state and other internal fields.

Author: dunxen

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -1436,12 +1436,12 @@ fn monitor_failed_no_reestablish_response() {
 	{
 		let mut node_0_per_peer_lock;
 		let mut node_0_peer_state_lock;
-		get_channel_ref!(nodes[0], nodes[1], node_0_per_peer_lock, node_0_peer_state_lock, channel_id).announcement_sigs_state = AnnouncementSigsState::PeerReceived;
+		get_channel_ref!(nodes[0], nodes[1], node_0_per_peer_lock, node_0_peer_state_lock, channel_id).context.announcement_sigs_state = AnnouncementSigsState::PeerReceived;
 	}
 	{
 		let mut node_1_per_peer_lock;
 		let mut node_1_peer_state_lock;
-		get_channel_ref!(nodes[1], nodes[0], node_1_per_peer_lock, node_1_peer_state_lock, channel_id).announcement_sigs_state = AnnouncementSigsState::PeerReceived;
+		get_channel_ref!(nodes[1], nodes[0], node_1_per_peer_lock, node_1_peer_state_lock, channel_id).context.announcement_sigs_state = AnnouncementSigsState::PeerReceived;
 	}
 
 	// Route the payment and deliver the initial commitment_signed (with a monitor update failure