concurrent collection gc

[SchrodingerZhu]

I am wandering whether deferred_free is capable to handle concurrent GC.

My idea was to mimic epoch gc but
instead of checking the garbage pile on each access, we can defer the free operation to mimalloc and let it free the garbage removed two epochs away.

Ideally it may improve the performance but I am not sure whether it will enlarge the memory pressure.

[playXE]

I'm working on this PR: #397 especially for this purpose. If it will get merged you could iterate your GCs mi_heap_t instance just fine to perform deferred free for lazy sweeping. If you need concurrent sweeping you do not really need deferred_free and I guess you could just walk the heap mi_heap_visit_blocks?