Merge rust-bitcoin/rust-bitcoin#920: Push key xonly

f27c4a5 Added push_x_only_key(..) and its test. (mpls)

Pull request description:

  **Issue**

  I can not use [`XOnlyPublicKey`](https://github.com/rust-bitcoin/rust-secp256k1/blob/ae985dd1911183c22bab9c9a199aca7b154c40f9/src/key.rs#L973) in my Scripts which prevents me from working with Taproot.

  **Cause**

  The current version of [`script::Builder`](https://github.com/rust-bitcoin/rust-bitcoin/blob/0a2d45de094c4c0a4861dc19314568684ac4d91c/src/blockdata/script.rs#L121) does not accept `XOnlyPublicKey`s.

  **Solution**

  So, I created a function `push_xkey(self, key: &XOnlyPublicKey)` based on the existing [`push_key`](https://github.com/rust-bitcoin/rust-bitcoin/blob/0a2d45de094c4c0a4861dc19314568684ac4d91c/src/blockdata/script.rs#L914) function. I also augmented an [existing test](https://github.com/rust-bitcoin/rust-bitcoin/blob/0a2d45de094c4c0a4861dc19314568684ac4d91c/src/blockdata/script.rs#L1108) in an attempt to reach testing parity with existing code.

  After toying around with `push_xkey`, it seems to work on my end.

ACKs for top commit:
  dr-orlovsky:
    ACK f27c4a5
  sanket1729:
    utACK f27c4a5. Thanks a lot for keeping up the iterations with prompt responses

Tree-SHA512: 064958d49edc1d3636a21e428d62c2e9bcd9b13bd226c5821db9e04ce78663a11fcf601c7667b564f88e845207219a052e1c7413f50e5d27c79003e8129825ed

Author: dr-orlovsky

src/blockdata/script.rs

@@ -43,7 +43,7 @@ use OutPoint;
 use util::key::PublicKey;
 use util::address::WitnessVersion;
 use util::taproot::{LeafVersion, TapBranchHash, TapLeafHash};
-use secp256k1::{Secp256k1, Verification};
+use secp256k1::{Secp256k1, Verification, XOnlyPublicKey};
 use schnorr::{TapTweak, TweakedPublicKey, UntweakedPublicKey};
 
 /// A Bitcoin script.
@@ -919,6 +919,11 @@ impl Builder {
         }
     }
 
+    /// Adds instructions to push an XOnly public key onto the stack.
+    pub fn push_x_only_key(self, x_only_key: &XOnlyPublicKey) -> Builder {
+        self.push_slice(&x_only_key.serialize())
+    }
+
     /// Adds a single opcode to the script.
     pub fn push_opcode(mut self, data: opcodes::All) -> Builder {
         self.0.push(data.into_u8());
@@ -1117,6 +1122,17 @@ mod test {
         script = script.push_opcode(opcodes::all::OP_CHECKSIG); comp.push(0xACu8); assert_eq!(&script[..], &comp[..]);
     }
 
+    #[test]
+    fn script_x_only_key() {
+        // Notice the "20" which prepends the keystr. That 20 is hexidecimal for "32". The Builder automatically adds the 32 opcode
+        // to our script in order to give a heads up to the script compiler that it should add the next 32 bytes to the stack.
+        // From: https://github.com/bitcoin-core/btcdeb/blob/e8c2750c4a4702768c52d15640ed03bf744d2601/doc/tapscript-example.md?plain=1#L43
+        let keystr = "209997a497d964fc1a62885b05a51166a65a90df00492c8d7cf61d6accf54803be";
+        let x_only_key = XOnlyPublicKey::from_str(&keystr[2..]).unwrap();
+        let script = Builder::new().push_x_only_key(&x_only_key);
+        assert_eq!(script.0, Vec::from_hex(keystr).unwrap());
+    }
+
     #[test]
     fn script_builder() {
         // from txid 3bb5e6434c11fb93f64574af5d116736510717f2c595eb45b52c28e31622dfff which was in my mempool when I wrote the test