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/override/malloc.cc

@@ -86,7 +86,7 @@ extern "C"
 #endif
     size_t sz = Alloc::alloc_size(ptr);
     // Keep the current allocation if the given size is in the same sizeclass.
-    if (sz == sizeclass_to_size(size_to_sizeclass(size)))
+    if (sz == round_size(size))
       return ptr;
 
     void* p = SNMALLOC_NAME_MANGLE(malloc)(size);