Provide `cfg(creusot)`

[LevitatingLion]

It would be great if Creusot provided a cfg(creusot) when compiling under cargo creusot.

This would remove the need for the contracts feature in creusot-contracts, since the Creusot-specific code could switch to its rustc-compatible mode on its own. It would also allow annotated rust code to be compiled both with normal rustc and under Creusot without adding a contracts feature to every crate that uses Creusot's annotations.

[xldenis]

Does rustc ignore unknown cfgs ? If so I'll see how we can add this

[LevitatingLion]

A quick test, looks like it does ignore unknown cfgs: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=78fd0d6ac7925a9749ef1efbc94c2379

[xldenis]

Great, I'll work on setting up Creusot to set this cfg then