Improve ChannelDetails readability significantly.

[TheBlueMatt]

After the merge of #984, Jeff pointed out that ChannelDetails has
become a bit of a "bag of variables", and that a few of the variable
names in #984 were more confusing than necessary in context.

This addresses several issues by:

• Splitting counterparty parameters into a separate
  ChannelCounterpartyParameters struct,
• using the name unspendable_punishment_reserve for both outbound
  and inbound channel reserves, differentiating them based on their
  position in the counterparty parameters struct or not,
• Using the name force_close_spend_delay instead of
  spend_csv_on_our_commitment_funds to better communicate what
  is occurring.

[codecov]

Codecov Report

Merging #988 (ceca6f4) into main (431f807) will decrease coverage by 0.01%.
The diff coverage is 90.24%.

❗ Current head ceca6f4 differs from pull request most recent head 2b08a47. Consider uploading reports for the commit 2b08a47 to get more accurate results

@@            Coverage Diff             @@
##             main     #988      +/-   ##
==========================================
- Coverage   90.73%   90.72%   -0.02%     
==========================================
  Files          60       60              
  Lines       30702    30656      -46     
==========================================
- Hits        27857    27812      -45     
+ Misses       2845     2844       -1     

Impacted Files	Coverage Δ
lightning-invoice/src/utils.rs	84.26% <0.00%> (ø)
lightning/src/ln/channelmanager.rs	84.72% <94.11%> (-0.02%)	⬇️
lightning/src/routing/router.rs	95.93% <95.45%> (-0.18%)	⬇️
lightning/src/ln/functional_tests.rs	97.30% <0.00%> (+0.04%)	⬆️

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[jkczyz]

Thanks for making this change!

[TheBlueMatt]

Squashed with no changes, only changes since Jeff's ACK are below. Will merge after CI.

$ git diff-tree -U1 a44b282 2b08a47e
diff --git a/fuzz/src/router.rs b/fuzz/src/router.rs
index f51c9ab7..baa32312 100644
--- a/fuzz/src/router.rs
+++ b/fuzz/src/router.rs
@@ -15,3 +15,3 @@ use lightning::chain;
 use lightning::chain::transaction::OutPoint;
-use lightning::ln::channelmanager::{ChannelDetails, ChannelCounterpartyParameters};
+use lightning::ln::channelmanager::{ChannelDetails, ChannelCounterparty};
 use lightning::ln::features::InitFeatures;
@@ -209,3 +209,3 @@ pub fn do_test<Out: test_logger::Output>(data: &[u8], out: Out) {
 								channel_id: [0; 32],
-								counterparty: ChannelCounterpartyParameters {
+								counterparty: ChannelCounterparty {
 									node_id: *rnid,
diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index ceae6d10..e38b232e 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -607,3 +607,5 @@ const CHECK_CLTV_EXPIRY_SANITY_2: u32 = MIN_CLTV_EXPIRY_DELTA as u32 - LATENCY_G
 #[derive(Clone, Debug, PartialEq)]
-pub struct ChannelCounterpartyParameters {
+pub struct ChannelCounterparty {
+	/// The node_id of our counterparty
+	pub node_id: PublicKey,
 	/// The Features the channel counterparty provided upon last connection.
@@ -612,4 +614,2 @@ pub struct ChannelCounterpartyParameters {
 	pub features: InitFeatures,
-	/// The node_id of our counterparty
-	pub node_id: PublicKey,
 	/// The value, in satoshis, that must always be held in the channel for our counterparty. This
@@ -636,3 +636,3 @@ pub struct ChannelDetails {
 	/// Parameters which apply to our counterparty. See individual fields for more information.
-	pub counterparty: ChannelCounterpartyParameters,
+	pub counterparty: ChannelCounterparty,
 	/// The Channel's funding transaction output, if we've negotiated the funding transaction with
@@ -1180,3 +1180,3 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 					channel_id: (*channel_id).clone(),
-					counterparty: ChannelCounterpartyParameters {
+					counterparty: ChannelCounterparty {
 						node_id: channel.get_counterparty_node_id(),
diff --git a/lightning/src/routing/router.rs b/lightning/src/routing/router.rs
index 0faee7be..6c14ebeb 100644
--- a/lightning/src/routing/router.rs
+++ b/lightning/src/routing/router.rs
@@ -1206,3 +1206,3 @@ mod tests {
 			channel_id: [0; 32],
-			counterparty: channelmanager::ChannelCounterpartyParameters {
+			counterparty: channelmanager::ChannelCounterparty {
 				features,
$