Introduce Ntxid

diff --git a/bitcoin/src/blockdata/transaction.rs b/bitcoin/src/blockdata/transaction.rs
index eb5a0dbd..8ac8ac2f 100644
--- a/bitcoin/src/blockdata/transaction.rs
+++ b/bitcoin/src/blockdata/transaction.rs
@@ -14,7 +14,6 @@ use core::fmt;
 
 #[cfg(feature = "arbitrary")]
 use arbitrary::{Arbitrary, Unstructured};
-use hashes::sha256d;
 use internals::{compact_size, const_casts, write_err, ToU64};
 use io::{BufRead, Write};
 use primitives::Sequence;
@@ -31,7 +30,7 @@ use crate::{internal_macros, Amount, FeeRate, SignedAmount};
 
 #[rustfmt::skip]            // Keep public re-exports separate.
 #[doc(inline)]
-pub use primitives::transaction::{OutPoint, ParseOutPointError, Transaction, Txid, Wtxid, Version, TxIn, TxOut};
+pub use primitives::transaction::{OutPoint, ParseOutPointError, Transaction, Ntxid, Txid, Wtxid, Version, TxIn, TxOut};
 
 internal_macros::impl_hashencode!(Txid);
 internal_macros::impl_hashencode!(Wtxid);
@@ -215,7 +214,7 @@ fn size_from_script_pubkey(script_pubkey: &Script) -> usize {
 pub trait TransactionExt: sealed::Sealed {
     /// Computes a "normalized TXID" which does not include any signatures.
     #[deprecated(since = "0.31.0", note = "use `compute_ntxid()` instead")]
-    fn ntxid(&self) -> sha256d::Hash;
+    fn ntxid(&self) -> Ntxid;
 
     /// Computes the [`Txid`].
     #[deprecated(since = "0.31.0", note = "use `compute_txid()` instead")]
@@ -334,7 +333,7 @@ pub trait TransactionExt: sealed::Sealed {
 }
 
 impl TransactionExt for Transaction {
-    fn ntxid(&self) -> sha256d::Hash { self.compute_ntxid() }
+    fn ntxid(&self) -> Ntxid { self.compute_ntxid() }
 
     fn txid(&self) -> Txid { self.compute_txid() }
 
diff --git a/primitives/src/lib.rs b/primitives/src/lib.rs
index 4aa1f323..c0a60c78 100644
--- a/primitives/src/lib.rs
+++ b/primitives/src/lib.rs
@@ -83,7 +83,7 @@ pub use self::{
     block::{BlockHash, Header as BlockHeader, Version as BlockVersion, WitnessCommitment},
     merkle_tree::{TxMerkleNode, WitnessMerkleNode},
     pow::CompactTarget,
-    transaction::{OutPoint, Txid, Version as TransactionVersion, Wtxid},
+    transaction::{Ntxid, OutPoint, Txid, Version as TransactionVersion, Wtxid},
 };
 
 #[rustfmt::skip]
diff --git a/primitives/src/transaction.rs b/primitives/src/transaction.rs
index 3ca18d15..0a916778 100644
--- a/primitives/src/transaction.rs
+++ b/primitives/src/transaction.rs
@@ -121,11 +121,16 @@ impl Transaction {
 
     /// Computes a "normalized TXID" which does not include any signatures.
     ///
-    /// This gives a way to identify a transaction that is "the same" as
-    /// another in the sense of having same inputs and outputs.
+    /// This function is needed only for legacy (pre-Segwit or P2SH-wrapped segwit version 0)
+    /// applications. This method clears the `script_sig` field of each input, which in Segwit
+    /// transactions is already empty, so for Segwit transactions the ntxid will be equal to the
+    /// txid, and you should simply use the latter.
+    ///
+    /// This gives a way to identify a transaction that is "the same" as another in the sense of
+    /// having the same inputs and outputs.
     #[doc(alias = "ntxid")]
-    pub fn compute_ntxid(&self) -> sha256d::Hash {
-        let cloned_tx = Transaction {
+    pub fn compute_ntxid(&self) -> Ntxid {
+        let normalized = Transaction {
             version: self.version,
             lock_time: self.lock_time,
             inputs: self
@@ -139,7 +144,7 @@ impl Transaction {
                 .collect(),
             outputs: self.outputs.clone(),
         };
-        sha256d::Hash::from_byte_array(cloned_tx.compute_txid().to_byte_array())
+        Ntxid::from_byte_array(normalized.compute_txid().to_byte_array())
     }
 
     /// Computes the [`Txid`].
@@ -486,14 +491,27 @@ hashes::hash_newtype! {
 
     /// A bitcoin witness transaction ID.
     pub struct Wtxid(sha256d::Hash);
+
+    /// A "normalized TXID".
+    ///
+    /// Computed on a transaction that has had the signatures removed.
+    ///
+    /// This type is needed only for legacy (pre-Segwit or P2SH-wrapped segwit version 0)
+    /// applications. This method clears the `script_sig` field of each input, which in Segwit
+    /// transactions is already empty, so for Segwit transactions the ntxid will be equal to the
+    /// txid, and you should simply use the latter.
+    ///
+    /// This gives a way to identify a transaction that is "the same" as another in the sense of
+    /// having the same inputs and outputs.
+    pub struct Ntxid(sha256d::Hash);
 }
 
 #[cfg(feature = "hex")]
-hashes::impl_hex_for_newtype!(Txid, Wtxid);
+hashes::impl_hex_for_newtype!(Txid, Wtxid, Ntxid);
 #[cfg(not(feature = "hex"))]
-hashes::impl_debug_only_for_newtype!(Txid, Wtxid);
+hashes::impl_debug_only_for_newtype!(Txid, Wtxid, Ntxid);
 #[cfg(feature = "serde")]
-hashes::impl_serde_for_newtype!(Txid, Wtxid);
+hashes::impl_serde_for_newtype!(Txid, Wtxid, Ntxid);
 
 impl Txid {
     /// The `Txid` used in a coinbase prevout.