Fix the sandbox use case and add a test. (#269)

Summary of changes:

- Add a new PAL that doesn't allocate memory, which can be used with a
  memory provider that is pre-initialised with a range of memory.
- Add a `NoAllocation` PAL property so that the methods on a PAL that 
  doesn't support dynamically reserving address space will never be
  called and therefore don't need to be implemented.
- Slightly refactor the memory provider class so that it has a narrower
  interface with LargeAlloc and is easier to proxy.
- Allow the address space manager and the memory provider to be
  initialised with a range of memory.

This may eventually also remove the need for (or, at least, simplify)
the Open Enclave PAL.

This commit also ends up with a few other cleanups:

 - The `malloc_useable_size` CMake test that checks whether the
   parameter is const qualified was failing on FreeBSD where this
   function is declared in `malloc_np.h` but where including
   `malloc.h` raises an error.  This should now be more robust.
 - The BSD aligned PAL inherited from the BSD PAL, which does not
   expose aligned allocation. This meant that it exposed both the
   aligned and non-aligned allocation interfaces and so happily
   accepted incorrect `constexpr` if blocks that expected one or 
   the other but accidentally required both to exist. The unaligned
   function is now deleted so the same failures that appear in CI should
   appear locally for anyone using this PAL.

src/mem/allocstats.h

@@ -196,6 +196,7 @@ namespace snmalloc
       UNUSED(sc);
 
 #ifdef USE_SNMALLOC_STATS
+      SNMALLOC_ASSUME(sc < LARGE_N);
       large_pop_count[sc]++;
 #endif
     }