Don't panic in `PrivateKey::from_slice`

During upgrade of `secp256k1` a number of function calls needed to be
rewritten to not use `from_slice` but `from_byte_array`. Unfortunately,
the conversions wasn't correct and caused panics on invalid inputs
rather than errors.

This fixes it simply by calling `try_into` on the entire slice and
converting the error.

bitcoin/src/crypto/key.rs

@@ -466,7 +466,7 @@ impl PrivateKey {
     ) -> Result<PrivateKey, secp256k1::Error> {
         Ok(PrivateKey::new(
             secp256k1::SecretKey::from_byte_array(
-                data[..32].try_into().expect("Slice should be exactly 32 bytes"),
+                data.try_into().map_err(|_| secp256k1::Error::InvalidSecretKey)?,
             )?,
             network,
         ))