Contracts on extern methods

[JamesFaix]

The first obvious answer is to ban contracts on extern methods. After all, the implementation is off in legacy code land.

However, it would be nice to be able to apply contracts to extern methods and have the contract implementation injected at callsites. Alternately, the extern method declaration could be renamed and a wrapper method could be generated to hold the contract implementations with the original method name.

[JamesFaix]

It would be nice to allow contracts on extern methods, in which the original method is made private and renamed, and a new public method is created with the original name, whose body implements the contracts and calls the inner method.