Fix a panic in Route's new fee-calculation methods and clean up

[TheBlueMatt]

This addresses Val's feedback on the new Route fee- and
amount-calculation methods, including fixing the panic she
identified and cleaning up various docs and comments.

See #1063 (review)

[codecov]

Codecov Report

Merging #1088 (59925c1) into main (8f78bbd) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1088      +/-   ##
==========================================
- Coverage   90.75%   90.72%   -0.04%     
==========================================
  Files          65       65              
  Lines       33948    34044      +96     
==========================================
+ Hits        30811    30886      +75     
- Misses       3137     3158      +21     

Impacted Files	Coverage Δ
lightning/src/routing/router.rs	96.00% <100.00%> (+<0.01%)	⬆️
lightning/src/ln/functional_tests.rs	97.50% <0.00%> (-0.04%)	⬇️
lightning/src/ln/peer_handler.rs	49.29% <0.00%> (+3.34%)	⬆️

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[TheBlueMatt]

Addressed some comments from Jeff and squashed, will land after CI, diff is:

$ git diff-tree -U1 6ccbe887 59925c1f
diff --git a/lightning/src/routing/router.rs b/lightning/src/routing/router.rs
index af783954..e60fd1ee 100644
--- a/lightning/src/routing/router.rs
+++ b/lightning/src/routing/router.rs
@@ -77,3 +77,3 @@ impl Route {
 	/// This doesn't include any extra payment made to the recipient, which can happen in excess of
-	/// the amount passed to [``get_route``]'s `final_value_msat`.
+	/// the amount passed to [`get_route`]'s `final_value_msat`.
 	pub fn get_total_fees(&self) -> u64 {
@@ -81,3 +81,3 @@ impl Route {
 		return self.paths.iter()
-			.flat_map(|path| path.split_last().map(|path_pfx| path_pfx.1).unwrap_or(&[]))
+			.flat_map(|path| path.split_last().map(|(_, path_prefix)| path_prefix).unwrap_or(&[]))
 			.map(|hop| &hop.fee_msat)
@@ -89,3 +89,3 @@ impl Route {
 		return self.paths.iter()
-			.map(|path| path.split_last().map(|hop| hop.0.fee_msat).unwrap_or(0))
+			.map(|path| path.split_last().map(|(hop, _)| hop.fee_msat).unwrap_or(0))
 			.sum();
$