disabled_contracts: a stub implementation of https://crates.io/crates/contracts

The contract macros are useful for development. However, it's important for performance sensitive code to obviously not incur any overhead from the annotations.

The stub macros in this crate help achieve that: code can switch between that and the real contracts crate depending on features, and the contract code does not even have to build anymore, so any helpers for the checking logic can also be conditionalised.