Make commented demo tests work again

[TimoGlastra]

Tests: https://github.com/hyperledger/anoncreds-rs/blob/main/tests/anoncreds_demos.rs

Need in memory storage implementation (current tests made a start at this)

• Identify what the (un)commented tests do/test
• Pick a list of tests that are important/where it makes sense
• Remove any commented code that doesn't test anything new

3000 of 3500 lines are still commented (85%). The tests are written for the Indy SDK, so we do have to update all code to work with AnonCreds RS. Goal is to reach a good level of testing, it's infeasible to update all tests from indy-sdk

[berendsliedrecht]

After #150 is merged, we are at 1080/2850 working (30%)