witness_version: Use Self in error From impl

We can use self in error `From` impls with no loss of clarity and it is
more maintainable.

bitcoin/src/blockdata/script/witness_version.rs

@@ -253,5 +253,5 @@ impl std::error::Error for Error {
 }
 
 impl From<bech32::Error> for Error {
-    fn from(e: bech32::Error) -> Error { Error::Bech32(e) }
+    fn from(e: bech32::Error) -> Self { Self::Bech32(e) }
 }