Add `from_byte_array` to `PrivateKey`.

Private keys have statically-known length of 32 bytes and we are
migrating types with known lenths to use `from_byte_array` methods. This
adds the method to `PrivateKey` as well and uses it to implement
`from_slice`.

bitcoin/src/crypto/key.rs

@@ -459,17 +459,21 @@ impl PrivateKey {
     /// Serializes the private key to bytes.
     pub fn to_vec(self) -> Vec<u8> { self.inner[..].to_vec() }
 
+    /// Deserializes a private key from a byte array.
+    pub fn from_byte_array(
+        data: [u8; 32],
+        network: impl Into<NetworkKind>
+    ) -> Result<PrivateKey, secp256k1::Error> {
+        Ok(PrivateKey::new(secp256k1::SecretKey::from_byte_array(&data)?, network))
+    }
+
     /// Deserializes a private key from a slice.
     pub fn from_slice(
         data: &[u8],
         network: impl Into<NetworkKind>,
     ) -> Result<PrivateKey, secp256k1::Error> {
-        Ok(PrivateKey::new(
+        let array = data.try_into().map_err(|_| secp256k1::Error::InvalidSecretKey)?;
-            secp256k1::SecretKey::from_byte_array(
+        Self::from_byte_array(array, network)
-                data.try_into().map_err(|_| secp256k1::Error::InvalidSecretKey)?,
-            )?,
-            network,
-        ))
     }
 
     /// Formats the private key to WIF format.