- each handle now has a 4-bits "age", meaning that handles are recycled
only after 16 alloc/free cycles.
This is used to detect double-free and use-after free.
This should also allow us to compare handles, because freeing and
reallocating an object, won't produce the same Handle (at least
for 16 rounds).
- removed "type safety" checks because it's almost impossible to
get it wrong thanks to our compile time type safety checks. This
didn't provide a useful value added.
- This feature is built on top of being able to set/get a 8 bits tag
associated with the memory block returned by the pool allocator. We
use the "extra" parameter of the allocator to allocate a "hidden"
structure containing the age of that memory block.
- Also we don't allow to compare Handle<> of different types
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