Enable Kani to verify the standard library crates

[celinval]

As we experiment with verifying the standard library, it might be helpful to enable users to verify the standard library crates directly, instead of having to create wrappers. Those wrappers can also only be used to verify public components.

There are a few things that need to be done in order to do that:

• Create a #[no_core] Kani library #3153
• Instrument contracts in core without proc_macros. I believe procedural macros are still allowed.
• Modify std fork to declare kani module using kani_core crate.
• Add an option to Kani to verify a custom version of the standard library.

[celinval]

See rust-lang/wg-cargo-std-aware#7 for workarounds on how to build a modified std library.