Macro kani_lib 

macro_rules! kani_lib {
    (core) => { ... };
    (kani) => { ... };
}

Users should only need to invoke this.

Options are:

• kani: Add definitions needed for Kani library.
• core: Define a kani module inside core crate.
• std: TODO: Define a kani module inside std crate. Users must define kani inside core.