Use non-mocked Ed25519 crypto

[ch1bo]

What & Why

As we have #193 only a bit later, we want to switch to using proper keys and signature scheme still using naiive multi-signatures consisting of a simple list of signatures.

We also want to measure how expensive it is to validate closeTx with n number of parties and using proper crypto. This will give us an estimate in how pressing using aggregated multi-signatures is.

Tasks

• Add CloseTx cost benchmark to tx-cost and similar test suites #322
• Introduce Hydra.Crypto module which uses ed25519 signatures and use it everywhere #326

[abailly-iohk]

Ironically enough, CloseTx is missing from our transaction costs benchmark so this issue would be a great motivator for adding it

[ch1bo]

Added it into a rough tasks list!