### Approach So far

The approach so far has been to shift the `kani` library's functionality
to `kani_core` instead, which has no dependencies of its own. By moving
these API functions to macros, we delay the compilation of these to when
Kani is invoked.

Tested by using the regression tests added @celinval in
#3236 using the new
`verify-std` subcommand.

### Running Kani

To test Kani itself, we injected a proof inside the core library by
making these changes.

```  rust
#[cfg(kani)]
kani_core::kani_lib!(core);

#[cfg(kani)]
#[unstable(feature = "kani", issue = "none")]
pub mod verify {
    use crate::kani;

    #[kani::proof]
    pub fn harness() {
        kani::assert(true, "yay");
    }
}
```

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

---------

Co-authored-by: Celina G. Val <celinval@amazon.com>