Rename schnorrsig method

Recently we deprecated a bunch of functions/methods that used the term
`schnorrsig`. Seems we left `generate_schnorrsig_keypair` in there,
along with some stall docs on it.

Rename `generate_schnorrsig_keypair` -> `generate_keypair_xonly`.
Deprecate the old one and improve the function docs while we are at it.

Author: tcharding

src/key.rs

@@ -1547,7 +1547,7 @@ mod test {
         for _ in 0..10 {
             let mut tweak = [0u8; 32];
             thread_rng().fill_bytes(&mut tweak);
-            let (mut kp, mut pk) = s.generate_schnorrsig_keypair(&mut thread_rng());
+            let (mut kp, mut pk) = s.generate_keypair_xonly(&mut thread_rng());
             let orig_pk = pk;
             kp.tweak_add_assign(&s, &tweak).expect("Tweak error");
             let parity = pk.tweak_add_assign(&s, &tweak).expect("Tweak error");

src/schnorr.rs

@@ -248,18 +248,35 @@ impl<C: Verification> Secp256k1<C> {
 }
 
 impl <C: Signing> Secp256k1<C> {
-
-    /// Generates a random Schnorr KeyPair and its associated Schnorr PublicKey.
-    /// Convenience function for `schnorrsig::KeyPair::new` and
-    /// `schnorrsig::PublicKey::from_keypair`; call those functions directly for
-    /// batch key generation. Requires a signing-capable context. Requires compilation
-    /// with the "rand" feature.
+    /// Generates a random Schnorr `KeyPair` and its associated Schnorr `XOnlyPublicKey`.
+    ///
+    /// Convenience function for `key::KeyPair::new` and `key::XOnlyPublicKey::from_keypair`; call
+    /// those functions directly for batch key generation.
+    ///
+    /// Requires a signing-capable context and requires compilation with the "rand" feature.
     #[inline]
     #[cfg(any(test, feature = "rand"))]
     #[cfg_attr(docsrs, doc(cfg(feature = "rand")))]
+    #[deprecated(since = "0.21.0", note = "Use generate_keypair_xonly instead.")]
     pub fn generate_schnorrsig_keypair<R: Rng + ?Sized>(
         &self,
         rng: &mut R,
+    ) -> (KeyPair, XOnlyPublicKey) {
+        self.generate_keypair_xonly(rng)
+    }
+
+    /// Generates a random Schnorr `KeyPair` and its associated Schnorr `XOnlyPublicKey`.
+    ///
+    /// Convenience function for `key::KeyPair::new` and `key::XOnlyPublicKey::from_keypair`; call
+    /// those functions directly for batch key generation.
+    ///
+    /// Requires a signing-capable context and requires compilation with the "rand" feature.
+    #[inline]
+    #[cfg(any(test, feature = "rand"))]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rand")))]
+    pub fn generate_keypair_xonly<R: Rng + ?Sized>(
+        &self,
+        rng: &mut R,
     ) -> (KeyPair, XOnlyPublicKey) {
         let sk = KeyPair::new(self, rng);
         let pubkey = XOnlyPublicKey::from_keypair(&sk);
@@ -325,7 +342,7 @@ mod tests {
         let secp = Secp256k1::new();
 
         let mut rng = thread_rng();
-        let (seckey, pubkey) = secp.generate_schnorrsig_keypair(&mut rng);
+        let (seckey, pubkey) = secp.generate_keypair_xonly(&mut rng);
         let mut msg = [0u8; 32];
 
         for _ in 0..100 {
@@ -390,7 +407,7 @@ mod tests {
     #[test]
     fn test_pubkey_serialize_roundtrip() {
         let secp = Secp256k1::new();
-        let (_, pubkey) = secp.generate_schnorrsig_keypair(&mut thread_rng());
+        let (_, pubkey) = secp.generate_keypair_xonly(&mut thread_rng());
         let ser = pubkey.serialize();
         let pubkey2 = XOnlyPublicKey::from_slice(&ser).unwrap();
         assert_eq!(pubkey, pubkey2);
@@ -513,7 +530,7 @@ mod tests {
         }
 
         let s = Secp256k1::new();
-        let (_, pubkey) = s.generate_schnorrsig_keypair(&mut DumbRng(0));
+        let (_, pubkey) = s.generate_keypair_xonly(&mut DumbRng(0));
         assert_eq!(
             &pubkey.serialize()[..],
             &[