A region allocator (a.k.a arena, area, zone) for D's allocator interface. This is std.experimental.allocator.building_blocks.region.Region with two modifications

• deallocate now does nothing, even if it's trying to deallocate the last allocation
• Added a simple reallocate implementation

Region allocators are best when used for short-lived tasks or objects that you know have a very short lifetime. For example, we know that all of the objects allocated when handling an HTTP request

To that end, the default implementation had two flaws. First, implementing deallocate at all goes against the idea of the relying on a fast batch deallocate when the task/job is over. Second, not having a reallocate implementation meant that it couldn't be used for arrays, only for linked lists.