Remove at_least

The Pal was providing policy for overallocating a block of memory to
achieve alignment make that part of the backend.
The backend should be responsible for layout policy.

src/backend/backend.h

@@ -11,7 +11,7 @@ namespace snmalloc
    * This class implements the standard backend for handling allocations.
    * It abstracts page table management and address space management.
    */
-  template<typename PAL, bool fixed_range>
+  template<SNMALLOC_CONCEPT(ConceptPAL) PAL, bool fixed_range>
   class BackendAllocator
   {
   public: