MemoryPool maintains a free list of pre-allocated entries stored in a
libcds MPMC Queue. In contrast to LocklessAllocator, it's a lot faster,
less (and less complex) code to maintain for us and it doesn't fail when
it's full.
Custom CMake module which attempts to automatically clone submodules when they're missing. Uses the `--depth` option if supported, which should be faster than running `--recursive` on initial clone.
