[Macro] Add `init` decorator so that init methods require that the subcontract state is None

[willemneal]

          I think I understand  - `&mut self` here opens us up to the possibility that the internal state of the singleton can be overwritten. 

Are you saying that you want to redesign this so that users aren't able to get themselves into that situation by initializing a contract with a state of None. And then other methods require that the state is Some? Otherwise it will indicate that the contract hasn't been initialized.

That all makes sense to me!

Originally posted by @elizabethengelman in #91 (comment)