Since we're now including jemalloc_internal.h, all the required headers
are already pulled. This will avoid having to fiddle with headers that can
or can't be used with MSVC. Also, now that we use malloc_printf, we can use
util.h's definition of assert instead of assert.h's.
Implement aligned_alloc(), which was added in the C11 standard. The
function is weakly specified to the point that a minimally compliant
implementation would be painful to use (size must be an integral
multiple of alignment!), which in practice makes posix_memalign() a
safer choice.
