Add intrinsics and Arbitrary support for no_core

[jaisnan]

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.

#[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.

[celinval]

Awesome! Thanks