bitcoin: Add default arg to bip32 example

Use the documented seed hex string as default if one is not passed in.
This allows us to simplify running this example in CI.

Author: tcharding

bitcoin/examples/bip32.rs

@@ -1,4 +1,4 @@
-use std::{env, process};
+use std::env;
 
 use bitcoin::address::{Address, KnownHrp};
 use bitcoin::bip32::{ChildNumber, DerivationPath, Xpriv, Xpub};
@@ -14,12 +14,12 @@ fn main() {
     // cargo run --example bip32 7934c09359b234e076b9fa5a1abfd38e3dc2a9939745b7cc3c22a48d831d14bd
 
     let args: Vec<String> = env::args().collect();
-    if args.len() < 2 {
-        eprintln!("not enough arguments. usage: {} <hex-encoded 32-byte seed>", &args[0]);
-        process::exit(1);
-    }
+    let seed_hex = if args.len() < 2 {
+        "7934c09359b234e076b9fa5a1abfd38e3dc2a9939745b7cc3c22a48d831d14bd"
+    } else {
+        &args[1]
+    };
 
-    let seed_hex = &args[1];
     println!("Seed: {seed_hex}");
     println!("Using mainnet network");