Without this, one could use freed objects without issues,
until something else was allocated in that space.  And even then,
it would still be a valid object.

So, in debug mode overwrite the data with garbage to try to surface
these issues.

This exposed an issue with our "nonheap roots" handling, where we
weren't scanning all of the memory that they pointed to.  This is
mostly fine, but there are some cases (time.gmtime) where gc-allocated
memory would be stored in these objects.  So, now you have to register
the size of the object, and the memory range will be scanned conservatively.