Merge #57: remove std feature gate on OutputUpdateError

e078b2e remove std feature gate on OutputUpdateError (Riccardo Casatta)

Pull request description:

  It seems this feature gate is a refuse (since for example is not on `UtxoUpdateError`) and not implementing std::error::Error cause issues downstream

ACKs for top commit:
  apoelstra:
    ACK e078b2e

Tree-SHA512: 8767be5e8f91b1ec3591056e05680f4bbd034a771ff2a3070c48fb0195b2f37358ddfe01b6f20c5ad5f1777984df0e26d8097e824a0d50ba15ac9d39e7ee5598

Author: apoelstra

src/psbt/mod.rs

@@ -1433,7 +1433,6 @@ impl fmt::Display for OutputUpdateError {
     }
 }
 
-#[cfg(feature = "std")]
 impl error::Error for OutputUpdateError {
     fn cause(&self) -> Option<&dyn error::Error> {
         use self::OutputUpdateError::*;