Partially integrate uninit memory checks into `verify_std`

[artemagvanian]

This PR partially integrates uninitialized memory checks into the verify_std pipeline, which makes it possible to enable them for the Rust Standard Library verification.

Changes:

• Move mem_init.rs library code into kani_core.
• Add a conditional compilation flag to disable uninitialized memory checks whenever some functionality is not yet supported.

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