Use Self:: in error return type

As is becoming customary in this codebase use `Self::Foo` to return the
error variant in `From` impl.

Refactor only, no logic changes.

bitcoin/src/crypto/sighash.rs

@@ -283,7 +283,7 @@ impl std::error::Error for Error {
 }
 
 impl From<io::Error> for Error {
-    fn from(e: io::Error) -> Self { Error::Io(e.kind()) }
+    fn from(e: io::Error) -> Self { Self::Io(e.kind()) }
 }
 
 impl From<PrevoutsSizeError> for Error {