Finish implementing initial Simulation interface

[gjtrowbridge]

This commit implements all of the methods from the initial Simulation interface.

Notes:

• Some logic is new, some logic was ported from notion-server/src/ethereum/mod.rs. In a future diff, notion-server/src/ethereum/mod.rs will be removed entirely, and the server logic in notion-server will instead point to eth2/simulation/.
• Some reasons for this move / refactor:
  • Libraries used in implementing eth2-specific types were not compatible with libraries used by notion-server (eg. Rocket).
  • It didn't make sense to have the simulation and the server wrapping it be tightly coupled long-term, this approach is much more modular, and eg. will support using the simulation in other contexts also (eg. other wrappers could use the underlying Simulation logic, devs could use the Simulation logic directly without going through a wrapper, etc).

Next Steps:

1. Update notion-server to use the Simulation logic in the eth2/simulation repository.
2. Remove Simulation logic from notion-server.
3. Verify that notion-server is working and satisfies all current user needs.
4. Write an SDK library that can be imported into 3rd party projects and which abstracts out communication with notion-server. This will allow other projects to easily communicate with notion-server without needing to "manually" write HTTP requests.
5. Continue updating the notion-server, SDK, and Simulation interfaces as-necessary as user needs change.

[jrhea]

This is fantastic. The code is super easy to follow and the tests demonstrate how the interface will be used. The simulation interface is definitely functional, but it would be convenient if I didn't necessarily have to know about SSZ and other spec types in order to interact with it.

For example, when calling create_shard_block() I have to know about the ssz type VariableList in order to create a ShardTransaction. You could make this easier by either changing the CreatShardBlock struct to this:

    pub struct CreateShardBlock {
        pub shard: Shard,
        pub shard_transactions: Vec<u8>,
    }

or providing a helper method called create_shard_transaction() that accepts some bytes and an index.

[gjtrowbridge]

Closing this because all of this is included in #27