rustfmt forwarding-related methods

[tnull]

In the hopes of speeding up the review process, we split out the rustfmt-related commits of #3891 (as requested).

[ldk-reviews-bot]

👋 Thanks for assigning @joostjager as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[joostjager]

I think good decision to get this refactor/reformat out of the way first.

[valentinewallace]

rustfmt CI check is failing

[tnull]

Addressed the outstanding comments and force-pushed with a minor adjustment:

> git diff-tree -U2 e7a06f417 359e17e14
diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index d590748c2..525ce013c 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -6879,25 +6879,19 @@ where
                                                                self.best_block.read().unwrap().height,
                                                        );
-                                                       let short_channel_id = $htlc.prev_hop.short_channel_id;
-                                                       let user_channel_id = $htlc.prev_hop.user_channel_id;
                                                        let counterparty_node_id = $htlc.prev_hop.counterparty_node_id;
-                                                       let channel_id = prev_channel_id;
-                                                       let outpoint = prev_funding_outpoint;
-                                                       let htlc_id = $htlc.prev_hop.htlc_id;
                                                        let incoming_packet_shared_secret =
                                                                $htlc.prev_hop.incoming_packet_shared_secret;
-                                                       let cltv_expiry = Some(cltv_expiry);
                                                        failed_forwards.push((
                                                                HTLCSource::PreviousHopData(HTLCPreviousHopData {
-                                                                       short_channel_id,
-                                                                       user_channel_id,
+                                                                       short_channel_id: $htlc.prev_hop.short_channel_id,
+                                                                       user_channel_id: $htlc.prev_hop.user_channel_id,
                                                                        counterparty_node_id,
-                                                                       channel_id,
-                                                                       outpoint,
-                                                                       htlc_id,
+                                                                       channel_id: prev_channel_id,
+                                                                       outpoint: prev_funding_outpoint,
+                                                                       htlc_id: $htlc.prev_hop.htlc_id,
                                                                        incoming_packet_shared_secret,
                                                                        phantom_shared_secret,
                                                                        blinded_failure,
-                                                                       cltv_expiry,
+                                                                       cltv_expiry: Some(cltv_expiry),
                                                                }),
                                                                payment_hash,