Allocator: Handle basic dead threads with unshared memory

[SirOlaf]

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Ref: https://github.com/nim-lang/Nim/issues/23361

Fixes the first example in referenced issue. The second example requires that pages can be inherited as creating a string allocates which then causes a cell to be moved into a foreign thread.

next and prev fields of SmallChunk are kept around for now as I suspect they will help for the more complicated case.