Add generic ContractInterface impl for contracts that use only one root ComposableState

[nemonical]

this PR resolves #1

[CLAassistant]

All committers have signed the CLA.

[sanity]

It doesn't hurt to support contracts that are a single ComposableState, however the purpose of ComposableState is to allow more complex states to be built up out of composable components - which is why this wasn't previously supported.

[nemonical]

however the purpose of ComposableState is to allow more complex states to be built up out of composable components

• I imagined that this would be done in a hierarchical way, so one particular ComposableState contains more ComposableStates. This way you could leverage this kind of a system to reduce boilerplate while allowing any level of complexity. Is this not what you were imagining?
• I suppose you could also build a contract that is a list of ComposableStates, though this could also be done with a Pair type which is just two ComposableStates in a trenchcoat or something similar.
• Would it be reasonable to be able to #[derive(ContractInterface)] on types constructed purely from ComposableStates?