Contracts mvp

[JustusAdam]

Description of changes:

Adds a basic implementation for function contracts.
The code generation is adapted from @monal's work last year though it now emits CBMC contracts instead.

It is adding the following capabilities:

• kani::requires and kani::ensures macros which contain side-effect free rust predicates over the function inputs and the return value (currently called result). These compile into their CBMC equivalents
• Implement old() function for contracts #2545
• Quantifiers for function contracts #2546

TODO

• Drop auto-harness
• Enforce unstable feature

Call-outs:

Testing:

• How is this change tested?

• Is this a refactor change?

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

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