Allow to use garbage collector only on the part of the heap.

[m4drat]

E.g. some part of memory is manually-controlled, and another using GcPtr and GC. So add new info bit to the chunk structure: IsControlledByGc.

[m4drat]

This feature is considered useless, and thus will be removed in future, because users will lose all the advantages of this allocator if they are using raw pointers. For example they will not be able to use GC (because this requires heap compacting which in turn requires updating pointers). Even if we will make some code changes to support GC + raw pointers memory management this will still be useless due to the fact, that we are not able to update values of raw pointers.