pkcs1: fixups for DecodeRsaPublicKey blanket impl (#916)

Changes the error type to `spki::Error`, which matches the error type
used in the `spki::DecodePublicKey` blanket impl:

https://docs.rs/spki/0.7.0/spki/trait.DecodePublicKey.html#impl-DecodePublicKey-for-T

Also removes the bounds on `DecodePublicKey` in the blanket impl of
`DecodeRsaPublicKey`, since they aren't actually used.

This is a breaking change but also a bugfix, as with the previous bounds
it's not possible to impl PKCS#1 support via these two blanket impls,
since the error types don't match. The plan is to merge this, publish
it, and yank the v0.7.0 release.

pkcs1/src/traits.rs

@@ -169,9 +169,9 @@ where
 }
 
 #[cfg(feature = "pkcs8")]
-impl<T: pkcs8::DecodePublicKey> DecodeRsaPublicKey for T
+impl<T> DecodeRsaPublicKey for T
 where
-    T: for<'a> TryFrom<pkcs8::SubjectPublicKeyInfoRef<'a>, Error = pkcs8::Error>,
+    T: for<'a> TryFrom<pkcs8::SubjectPublicKeyInfoRef<'a>, Error = pkcs8::spki::Error>,
 {
     fn from_pkcs1_der(public_key: &[u8]) -> Result<Self> {
         Ok(Self::try_from(pkcs8::SubjectPublicKeyInfoRef {