Separate the hash_enum BCS parameter into hashchain enum and merkle tree enum

[alexander-zw]

The bcs_transformation_parameters object has an attribute named hash_enum. However, this is incorrect, since the BCS transformation is allowed to have different hash types for its hashchain and merkle tree. This attribute should be split into two different ones, perhaps called hashchain_hash_enum and merkle_hash_enum.

The hash_enum parameter is used to determine the default hash type, as well as for debugging purposes.