Crates using skeptic are recompiled no matter what

[torkleyy]

Even though I didn't modify anything, the crate gets recompiled. I don't think I'm the only one facing this issue: kvark/froggy#26

[steveatinfincia]

Just noticed this as well, we have a Visual Studio project that includes several Rust/Cargo sub-projects. Each one uses a crate that depends on skeptic, so even though nothing has changed in the Rust code or the dependencies, it take a full minute for each of them to build before the main project can build+run during development.

Perhaps a quick fix for crates using skeptic would be to put it behind a default feature flag that can be disabled, though I'm not sure what the root cause is.

[bozaro]

Root cause: rust-lang/cargo#4213