descriptors: add a method to get a PSBT's change outputs

[darosior]

Fixes #857.

[edouardparis]

utACK c86e6b6

nits:
Maybe would be nice to have a pub fn index(&self) -> usize method for ChangeOutput.
Maybe change index for vout because it can be confused with derivation index.

[darosior]

Maybe would be nice to have a pub fn index(&self) -> usize method for ChangeOutput.

Done:

diff --git a/src/descriptors/mod.rs b/src/descriptors/mod.rs
index b04dc7f..cc88645 100644
--- a/src/descriptors/mod.rs
+++ b/src/descriptors/mod.rs
@@ -124,6 +124,17 @@ pub enum ChangeOutput {
     DepositAddress { index: usize },
 }
 
+impl ChangeOutput {
+    /// Get the index of the change output in the transaction's list of outputs regardless of its
+    /// type.
+    pub fn index(&self) -> usize {
+        match self {
+            Self::ChangeAddress { index } => *index,
+            Self::DepositAddress { index } => *index,
+        }
+    }
+}
+
 impl LianaDescriptor {
     pub fn new(spending_policy: LianaPolicy) -> LianaDescriptor {
         // Get the descriptor from the chosen spending policy.

Maybe change index for vout because it can be confused with derivation index.

I think vout is equally confusing if not more. It's just a peculiar heritage of Satoshi's usage of hungarian notation in C++ (Vout stands for Vector of outputs).

[darosior]

ACK 3929926 -- re-applying Edouard's ACK. Only change is the addition of an index() method on the ChangeOutput enum.