Add Script conversion method p2wpkh_script_code

In order to sign a utxo that does a p2wpkh spend we need to create the
script that can be used to create a sighash. In the libbitcoin docs this
is referred to as the 'script code' [0].

The script is the same as a p2pkh script but the pubkey_hash is found in
the scriptPubkey.

Add a `Script` conversion method that checks if `self` is a v0 p2wpkh
script and if so extracts the pubkey_hash and returns the required
script.

[0] https://github.com/libbitcoin/libbitcoin-system/wiki/P2WPKH-Transactions#spending-a-p2wpkh-output

src/blockdata/script.rs

@@ -427,6 +427,25 @@ impl Script {
         Script::new_p2sh(&self.script_hash())
     }
 
+    /// Returns the script code used for spending a P2WPKH output if this script is a script pubkey
+    /// for 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(&self) -> Option<Script> {
+        if !self.is_v0_p2wpkh() {
+            return None
+        }
+        let script = Builder::new()
+            .push_opcode(opcodes::all::OP_DUP)
+            .push_opcode(opcodes::all::OP_HASH160)
+            .push_slice(&self[2..]) // The `self` script is 0x00, 0x14, <pubkey_hash>
+            .push_opcode(opcodes::all::OP_EQUALVERIFY)
+            .push_opcode(opcodes::all::OP_CHECKSIG)
+            .into_script();
+
+        Some(script)
+    }
+
     /// Computes the P2WSH output corresponding to this witnessScript (aka the "witness redeem
     /// script").
     pub fn to_v0_p2wsh(&self) -> Script {