Large alloc fix (#178)

* Improved malloc style tests

Added comprehensive testing of realloc, and other minor improvements
to reporting errors.

* Fix realloc resizing for large sizeclasses.

The rounding by sizeclass was incorrect for large allocation.  This fixes
that.

* Ensure alloc_size is committed

There is an awkward interaction between alloc_size and
committing only what is requested.  If the user assumes
everything up to alloc_size is available, then we need to
either store the more precise size for alloc_size to return
or commit the whole 2^n range, so that alloc_size stays simple.

This changes to just make the whole range committed.
In the future, we might want to store a more precise size, so
that the allocation can be sized more precisely.

* Reduce size of objects.

src/test/func/memory/memory.cc

@@ -221,7 +221,7 @@ void test_external_pointer_large()
 
   for (size_t i = 0; i < count; i++)
   {
-    size_t b = snmalloc::bits::is64() ? 28 : 26;
+    size_t b = SUPERSLAB_BITS + 3;
     size_t rand = r.next() & ((1 << b) - 1);
     size_t size = (1 << 24) + rand;
     total_size += size;