Add wrap constructor to V1/V2 StakeCredential

[solidsnakedev]

I'm not entirely sure if this is the right approach, but I wanted to suggest a potential solution to the missing constructor when a V1/V2 validator Purpose is of the Rewarding type. In such cases, the StakingCredential type must be wrapped in a StakingHash constructor as per the Plutus Ledger API for V1/V2:
Previous Aiken versions used to encode the StakeCredential as follows:
121([_ 122([_ h\'524A8B6C686BF0C225C8BC8EB63100BDF1047A35599626878EDF3BFE\'])])

For V3, however, this extra constructor in the Rewarding Purpose is not required , which seems to be correctly reflected in the latest Aiken version:

I'd also suggest renaming StakeCredential to Credential, as the current name can be misleading.

pub enum StakeCredential {
    AddrKeyhash(AddrKeyhash),
    Scripthash(Scripthash),
}

[KtorZ]

Duh, that's an oversight as you can see it's being done in every other places. And since we got rid of the acceptance tests for V2 recently, this has slipped through the cracks 😬 ...

Thanks.