Allocate slab is randomly in all possible permutations. This increases
the entropy of the order considerably. This uses an algorithm to build
a random cycle in a slab, and then use this to build the free list.
We disable the per-slab randomisation in the non-CHECK_CLIENT builds.
Define various parts of random that can be used to make the layout of
memory more random. Thread this through the allocator.
Expose the concept as part of the Pal. Subsequent commits will expose
that on different platforms.
