Investigate generalizing the GC so that it can be called anywhere

[bandurvp]

As the C code generator is increasingly targeted at applications outside FMU export, it becomes important to stop relying on the cyclic nature of INTO-CPS controller models.

Garbage collectors in general are build for specific architectures, which enables efficient determiation of whether variables are in scope. In the general case we can keep track of scope directly in the GC with explicit calls to "open" and "close" scope functions. The results can be correlated to entries in the GC data structures and it may be possible to determine variable scope in this way.