With this change storage keys will have a hash based on:
`sha256("storage::<storage_field_name1>::<storage_field_name2>")`
or a given key.

A key can be provide by using the `in` keyword in a storage field as such:
```
storage {
    a in 0x02dac99c283f16bc91b74f6942db7f012699a2ad51272b15207b9cc14a70dbae: u64
}
```

StorageKey.field_id is now based on:
`sha256("storage::<storage_field_name1>::<storage_field_name2>.<struct_field_name1>.<struct_field_name2>")`

Nested storage field such as the following are not yet supported and will be added in a later pull request.
```
storage {
    a {
        b:u64
    }
}
```