Meta issue: use abstract types instead of raw types

We should be more careful when using types around the code: for example, some parts of the code uses string or int directly to represent values. The downside is that we can invert values in function calls that expects multiple strings for example, which could have been avoided using abstract types to represent the values. Abstract types can also help to enforce invariants, which is pretty useful.

This meta issue tries to track such defects for a latter refactoring.

Abstract type for hashes

• secret
• secret keys and public keys
• operation hash