This allows us to finely control when to use a single hash or a double hash in
various places. For example, Bitcoin signatures use double SHA256, but Decred
signatures use a single BLAKE256. However, both use double hashes for Base58.
* Split ecdsa_curve into curve_info and ecdsa_curve to support bip32 on
curves that don't have a ecdsa_curve.
* Don't fail in key derivation but retry with a new hash.
* Adapted test case accordingly