model check helpers

This library provides some simple helpers for model checking using CBMC. Specifically, this library provides the MODEL_ASSERT macro which allows static assertions to be added to a program or library that are only evaluated at model checking time. Additionally, this library provides some default shadowing capabilities to inject slightly different behavior than is used by CBMC. For instance, a non-deterministic malloc is provided which may fail and return NULL. As this failure case is possible in systems with aggressive ulimit rules or embedded constraints, it is often desirable to model cases where dynamic memory allocation fails.