Enable stubbing and function contracts for primitive types

[celinval]

Previously, Kani was not able to resolve methods that belong to primitive types, which would not allow users to specify primitive types as stub target, neither for contracts.

Thus, extend the name resolution to be able to convert a type expression into a Ty for primitive types, and add a new method to search for function implementation for primitive types.

I also moved the type resolution logic to its own module to make it a bit cleaner.

Resolves #2646

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

[carolynzech]

Nice! You may want to add tests for the other PrimitiveIdent variants (bool and floats) for the sake of being exhaustive, but if you're confident enough that the logic is the same then I'm good either way.