Natural alignment for USE_MALLOC (#248)

* Add concept of natural alignment to tests.

snmalloc naturally aligns blocks very heavily, so that
the largest power-of-two in the rounded size is the alignment.
This checks that in the test, and provides a method for
finding the natural alignment of a block.

* Improve USE_MALLOC to provide alignment

snmalloc provides a lot of alginment guarantees. This ensures that when
we pass through to the system allocator we still get those alignment
guarantees.

The commit also fixes the tests to work with USE_MALLOC, and builds a
set of unit tests for ctest to check behaviour.

src/mem/globalalloc.h

@@ -72,7 +72,7 @@ namespace snmalloc
 
     void cleanup_unused()
     {
-#ifndef USE_MALLOC
+#ifndef SNMALLOC_PASS_THROUGH
       // Call this periodically to free and coalesce memory allocated by
       // allocators that are not currently in use by any thread.
       // One atomic operation to extract the stack, another to restore it.
@@ -102,7 +102,7 @@ namespace snmalloc
      */
     void debug_check_empty(bool* result = nullptr)
     {
-#ifndef USE_MALLOC
+#ifndef SNMALLOC_PASS_THROUGH
       // This is a debugging function. It checks that all memory from all
       // allocators has been freed.
       auto* alloc = Parent::iterate();