refactor(wallet)!: Make Wallet require a change descriptor

All `Wallet` constructors are modified to require a change
descriptor, where previously it was optional. Additionally
we enforce uniqueness of the change descriptor to avoid
ambiguity when deriving scripts and ensure the wallet will
always have two distinct keystores.

Notable changes

* Add error DescriptorError::ExternalAndInternalAreTheSame
* Remove error CreateTxError::ChangePolicyDescriptor
* No longer rely on `map_keychain`

example-crates/wallet_rpc/src/main.rs

@@ -25,7 +25,7 @@ pub struct Args {
     pub descriptor: String,
     /// Wallet change descriptor
     #[clap(env = "CHANGE_DESCRIPTOR")]
-    pub change_descriptor: Option<String>,
+    pub change_descriptor: String,
     /// Earliest block height to start sync from
     #[clap(env = "START_HEIGHT", long, default_value = "481824")]
     pub start_height: u32,
@@ -88,7 +88,7 @@ fn main() -> anyhow::Result<()> {
     let start_load_wallet = Instant::now();
     let mut wallet = Wallet::new_or_load(
         &args.descriptor,
-        args.change_descriptor.as_ref(),
+        &args.change_descriptor,
         Store::<bdk_wallet::wallet::ChangeSet>::open_or_create_new(
             DB_MAGIC.as_bytes(),
             args.db_path,