What will hopfully be the final version of the Backend interface. This
combines the MultiState interface into Backend since it will be required
to implement, and simplifies the interface because the Backend is no
longer responsible for tracking the current state.
Split the interface to change environments out from the minimal Backend
interface, to make it optional for backend implementations. If
backend.MultiState isn't implemented, return a "not implemented" from
environment related methods.
Have the Local backend delegate the MultiState methods to the proper
backend.
Backends are a mechanism that allow abstracting the behavior of
Terraform CLI from the actual core. This allows us to slip in special
behavior such as state loading, remote operations, etc.
James Bardin