feat(core): add is_all_zeros() and is_null() methods

Add convenience methods for identifying null outpoints and zero txids:
- TxidExt::is_all_zeros() checks if a txid consists of all zero bytes
- TxOutPointExt::is_null() identifies coinbase outpoints(index u32::MAX
  with all-zero txid)

Author: alexanderwiederin

src/core/transaction.rs

@@ -553,6 +553,11 @@ pub trait TxOutPointExt: AsPtr<btck_TransactionOutPoint> {
         let ptr = unsafe { btck_transaction_out_point_get_txid(self.as_ptr()) };
         unsafe { TxidRef::from_ptr(ptr) }
     }
+
+    /// Returns true if this OutPoint is the "null" coinbase OutPoint.
+    fn is_null(&self) -> bool {
+        self.index() == u32::MAX && self.txid().is_all_zeros()
+    }
 }
 
 /// A reference to a specific output in a previous transaction.
@@ -655,6 +660,11 @@ pub trait TxidExt: AsPtr<btck_Txid> {
         }
         bytes
     }
+
+    /// Returns true if all bytes of the txid are zero (null txid).
+    fn is_all_zeros(&self) -> bool {
+        self.to_bytes().iter().all(|&b| b == 0)
+    }
 }
 
 pub struct Txid {