Add public functions p2wpkh_script_code

Add two public API functions on the two public keys, both called
`p2wpkh_script_code` to do exactly as the name suggests.

Of note, I was not able to find anywhere to use these in example code,
this is because of we always use the new `p2wpkh_signature_hash`
function. The new functions may be useful for a user calling
`segwit_v0_encode_signing_data_to`. The may help document the library as
well.

bitcoin/src/blockdata/script/borrowed.rs

@@ -19,7 +19,7 @@ use crate::blockdata::script::{
     ScriptHash, WScriptHash,
 };
 use crate::consensus::Encodable;
-use crate::key::{PublicKey, UntweakedPublicKey};
+use crate::key::{PublicKey, UntweakedPublicKey, WPubkeyHash};
 use crate::policy::DUST_RELAY_TX_FEE;
 use crate::prelude::*;
 use crate::taproot::{LeafVersion, TapLeafHash, TapNodeHash};
@@ -385,14 +385,8 @@ impl Script {
     /// [BIP143]: <https://github.com/bitcoin/bips/blob/99701f68a88ce33b2d0838eb84e115cef505b4c2/bip-0143.mediawiki>
     pub fn p2wpkh_script_code(&self) -> Option<ScriptBuf> {
         self.p2wpkh().map(|wpkh| {
-            Builder::new()
+            let wpkh = WPubkeyHash::from_slice(wpkh).expect("checked in p2wpkh()");
-                .push_opcode(OP_DUP)
+            ScriptBuf::p2wpkh_script_code(wpkh)
-                .push_opcode(OP_HASH160)
-                // The `self` script is 0x00, 0x14, <pubkey_hash>
-                .push_slice(wpkh)
-                .push_opcode(OP_EQUALVERIFY)
-                .push_opcode(OP_CHECKSIG)
-                .into_script()
         })
     }
 

bitcoin/src/blockdata/script/owned.rs

@@ -151,6 +151,21 @@ impl ScriptBuf {
         Builder::new().push_opcode(version.into()).push_slice(program).into_script()
     }
 
+    /// Creates the script code used for spending a P2WPKH output.
+    ///
+    /// The `scriptCode` is described in [BIP143].
+    ///
+    /// [BIP143]: <https://github.com/bitcoin/bips/blob/99701f68a88ce33b2d0838eb84e115cef505b4c2/bip-0143.mediawiki>
+    pub fn p2wpkh_script_code(wpkh: WPubkeyHash) -> ScriptBuf {
+        Builder::new()
+            .push_opcode(OP_DUP)
+            .push_opcode(OP_HASH160)
+            .push_slice(wpkh)
+            .push_opcode(OP_EQUALVERIFY)
+            .push_opcode(OP_CHECKSIG)
+            .into_script()
+    }
+
     /// Generates OP_RETURN-type of scriptPubkey for the given data.
     pub fn new_op_return<T: AsRef<PushBytes>>(data: T) -> Self {
         Builder::new().push_opcode(OP_RETURN).push_slice(data).into_script()

bitcoin/src/crypto/key.rs

@@ -15,6 +15,7 @@ use internals::array_vec::ArrayVec;
 use internals::write_err;
 use io::{Read, Write};
 
+use crate::blockdata::script::ScriptBuf;
 use crate::crypto::ecdsa;
 use crate::internal_macros::impl_asref_push_bytes;
 use crate::network::NetworkKind;
@@ -71,6 +72,12 @@ impl PublicKey {
         }
     }
 
+    /// Returns the script code used to spend a P2WPKH input.
+    pub fn p2wpkh_script_code(&self) -> Result<ScriptBuf, UncompressedPubkeyError> {
+        let key = CompressedPublicKey::try_from(*self)?;
+        Ok(key.p2wpkh_script_code())
+    }
+
     /// Write the public key into a writer
     pub fn write_into<W: Write + ?Sized>(&self, writer: &mut W) -> Result<(), io::Error> {
         self.with_serialized(|bytes| writer.write_all(bytes))
@@ -266,6 +273,11 @@ impl CompressedPublicKey {
         WPubkeyHash::from_byte_array(hash160::Hash::hash(&self.to_bytes()).to_byte_array())
     }
 
+    /// Returns the script code used to spend a P2WPKH input.
+    pub fn p2wpkh_script_code(&self) -> ScriptBuf {
+        ScriptBuf::p2wpkh_script_code(self.wpubkey_hash())
+    }
+
     /// Write the public key into a writer
     pub fn write_into<W: io::Write + ?Sized>(&self, writer: &mut W) -> Result<(), io::Error> {
         writer.write_all(&self.to_bytes())