[Issue 18847] std.allocator: Region uses .parent before it can be set

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18847

--- Comment #1 from Vladimir Panteleev <dlang-bugzilla at thecybershadow.net> ---
Some goes for BitmappedBlock and KKRegion.

ContiguousFreeList seems to have gotten this right (though with the end result
of having 8 constructor declarations).

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