ByteBuffer allocator should use malloc_good_size() on Darwin

[weissi]

Darwin has a useful function malloc_good_size() which returns a good size to malloc. ByteBuffer's allocator should use that instead of just rounding up to the next power of 2.

The malloc_good_size() function rounds size up to a value that the allocator implementation can allocate without adding any padding; it then
returns that rounded-up value.

On Linux, we should also have a better strategy.