Optionaly set memory to zero when allocating

In addition to the `atomic` arg, there could be a `clear` arg to set the memory to zero on allocation. While we can call `memset` right after a `GC_malloc` it might be slightly more efficient to call it right when we allocate?

Or maybe not: we're likely to dereference the pointer right after we allocate, so calling memset when we allocate or when we dereference the pointer is maybe irrelevant.