[BumpPtrAllocator] Move DefaultSlabAllocator to a member of BumpPtrAllocator, instead of a static variable.

The problem with having DefaultSlabAllocator being a global static is that it is undefined if BumpPtrAllocator
will be usable during global initialization because it is not guaranteed that DefaultSlabAllocator will be
initialized before BumpPtrAllocator is created and used.