Implement with_state_drain & with_state_at_drain

[mustermeiszer]

The method should allow using any state and return the respective storage changes of the closure afterwards.

API

fn with_state_drain(exec: F) -> Result<(R, StorageChanges), Error> 
    where F: FnOnce() -> R {}

fn with_state_at_drain(at: BlockId, exec: F) -> Result<(R, StorageChanges), Error> 
    where F: FnOnce() -> R {}