`hevm` t8n support

Since [t8n](https://github.com/ethereum/go-ethereum/tree/master/cmd/evm) is now the canonical format for creating cross-client state tests, it would be great for `hevm` to allow for the transition to be defined in this way. The interface of `alloc.json`, `txs.json`, and `env.json` is nice and will only have more tooling built around it.

At minimum, it would be amazing to have a way to set the state via json instead of a directory structure. :)

Start with the hevm transition interface and compare it with the t8n inputs named in the issue: alloc.json, txs.json, and env.json. Determine how those files should replace the existing directory structure, and confirm that a complete state transition can be expressed and consumed through the JSON interface.

Written by the indexing model from the issue text.