Alloc #[cfg()] attributes on trait impls in mock!

[asomers]

mock! allows individual methods to be gated by #[cfg()]. It would be handy if that worked for trait impls too. For example,

mock! {
    struct Foo { }
    #[cfg(unix)]
    impl std::os::unix::io::AsRawFd for Foo {
        //...
    }
    #[cfg(windows)]
    impl std::os::windows::fs::FileExt for Foo {
        //...
    }
}

We certainly don't want both traits to be compiled at the same time.