New initial distributions with datums and reference scripts

[mmontin]

This PR introduces a richer initial distribution. The old initial distribution would only allow for Wallet to possess utxos containing values. The new initial distribution is extended to welcome any kind of initial payments. In particular, this includes:

• paying to a script
• attaching datums to initial utxos
• attaching reference scripts to initial utxos

The old helper to generate an initial distribution from wallets and values still exists, as this is the main use case. However, these new features would allow for instance to initialize oracles or have pre-existing reference scripts to be used when spending said scripts.

In addition, this PR introduces a module Cooked.Validators which contains dummy validators and associated datum types to be used in testing or redirecting outputs.

[florentc]

Reviewed during meeting.