bugfix: straighten out STL vector allocation (snapshotted vs. persistent)

When using a STL data structure allocated on the stack, we must make sure its
elements are allocated with the same allocator as the structure. For instance,
in a model-checking context, the stack is persistent (non-snapshotting), so any
stack-allocated std::vector should use the non-snapshotting allocator (i.e.,
MyAlloc).

At the same time, clarify CycleGraph and CycleNode classes by labelling them as
SNAPSHOTALLOC.