[$] The XArray data structure

Sometimes, a data structure proves to be inadequate for its intended task.Other times, though, the problem may be somewhere else - in the API used toaccess it, for example. Matthew Wilcox's presentation during the 2018linux.conf.au Kernel miniconf made the case that, for the kernel'svenerable radix tree data structure, thelatter situation holds. His response is a new approach to an old datastructure that he is calling the "XArray".