Add must_use for mut self key manipulation methods

We recently added a bunch of key tweaking methods that take `mut self`
and return the tweaked/negated keys. These functions are pure and as
such the returned result is expected to be used. To help downstream
users use the API correctly add `must_use` attributes with a descriptive
error string for each of the methods that takes `mut self`.

src/key.rs

@@ -243,6 +243,7 @@ impl SecretKey {
 
     /// Negates the secret key.
     #[inline]
+    #[must_use = "you forgot to use the negated secret key"]
     pub fn negate(mut self) -> SecretKey {
         unsafe {
             let res = ffi::secp256k1_ec_seckey_negate(
@@ -272,6 +273,7 @@ impl SecretKey {
     ///
     /// Returns an error if the resulting key would be invalid.
     #[inline]
+    #[must_use = "you forgot to use the tweaked secret key"]
     pub fn add_tweak(mut self, tweak: &Scalar) -> Result<SecretKey, Error> {
         unsafe {
             if ffi::secp256k1_ec_seckey_tweak_add(
@@ -302,6 +304,7 @@ impl SecretKey {
     ///
     /// Returns an error if the resulting key would be invalid.
     #[inline]
+    #[must_use = "you forgot to use the tweaked secret key"]
     pub fn mul_tweak(mut self, tweak: &Scalar) -> Result<SecretKey, Error> {
         unsafe {
             if ffi::secp256k1_ec_seckey_tweak_mul(
@@ -536,6 +539,7 @@ impl PublicKey {
 
     /// Negates the public key.
     #[inline]
+    #[must_use = "you forgot to use the negated public key"]
     pub fn negate<C: Verification>(mut self, secp: &Secp256k1<C>) -> PublicKey {
         unsafe {
             let res = ffi::secp256k1_ec_pubkey_negate(secp.ctx, &mut self.0);
@@ -566,6 +570,7 @@ impl PublicKey {
     ///
     /// Returns an error if the resulting key would be invalid.
     #[inline]
+    #[must_use = "you forgot to use the tweaked public key"]
     pub fn add_exp_tweak<C: Verification>(
         mut self,
         secp: &Secp256k1<C>,
@@ -602,6 +607,7 @@ impl PublicKey {
     ///
     /// Returns an error if the resulting key would be invalid.
     #[inline]
+    #[must_use = "you forgot to use the tweaked public key"]
     pub fn mul_tweak<C: Verification>(
         mut self,
         secp: &Secp256k1<C>,
@@ -971,6 +977,7 @@ impl KeyPair {
     /// ```
     // TODO: Add checked implementation
     #[inline]
+    #[must_use = "you forgot to use the tweaked key pair"]
     pub fn add_xonly_tweak<C: Verification>(
         mut self,
         secp: &Secp256k1<C>,
@@ -1270,6 +1277,7 @@ impl XOnlyPublicKey {
     /// let tweaked = xonly.add_tweak(&secp, &tweak).expect("Improbable to fail with a randomly generated tweak");
     /// # }
     /// ```
+    #[must_use = "you forgot to use the tweaked xonly pubkey"]
     pub fn add_tweak<V: Verification>(
         mut self,
         secp: &Secp256k1<V>,