Add push_p2wpkh function on Witness

In order to create the witness to spend p2wpkh output one must push the
signature and the pubkey, we should have a function for this.

bitcoin/src/blockdata/witness.rs

@@ -319,6 +319,25 @@ impl Witness {
         self.content[end_varint..end_varint + new_element.len()].copy_from_slice(new_element);
     }
 
+    /// Pushes data onto the witness stack as required to spend a P2WPKH output.
+    ///
+    /// Pushes the signature+sighash_type followed by the serialized public key. Also useful for
+    /// spending a P2SH-P2WPKH output.
+    ///
+    /// Note that it is expected that `signature`, `hash_type`, and `pubkey` are related.
+    ///
+    /// - `pubkey` is related to the private key that create `signature`.
+    /// - `hash_type` was used when creating the sighash signed to create `signature`.
+    pub fn push_p2wpkh(
+        &mut self,
+        signature: &ecdsa::Signature,
+        hash_type: EcdsaSighashType,
+        pubkey: &secp256k1::PublicKey,
+    ) {
+        self.push_bitcoin_signature(signature, hash_type);
+        self.push_slice(&pubkey.serialize());
+    }
+
     /// Pushes, as a new element on the witness, an ECDSA signature with the sighash type
     /// concatenated, requires an allocation.
     ///
@@ -527,6 +546,21 @@ impl From<Vec<&[u8]>> for Witness {
     fn from(vec: Vec<&[u8]>) -> Self { Witness::from_slice(&vec) }
 }
 
+/// Tried to use a legacy uncompressed pubkey in a segwit v0 transaction.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct UncompressedPubkeyError;
+
+impl fmt::Display for UncompressedPubkeyError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "segwit v0 requires a compressed pubkey")
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for UncompressedPubkeyError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
+
 #[cfg(test)]
 mod test {
     use secp256k1::ecdsa;