Is true iff T is an allocator, otherwise false.
Only members and alignment and allocate are required.
https://dlang.org/phobos/std_experimental_allocator_building_blocks.html
https://forum.dlang.org/post/uiavxptceyxsjulkxlec@forum.dlang.org
TODO: Move to Phobos.
See Implementation
Is true iff T is an allocator, otherwise false.
Only members and alignment and allocate are required.