This patch eliminates heap allocation in the VFS FAT disk IO layer, when
calling the underlying readblocks/writeblocks methods. The bytearray
object that is passed to these methods is now allocated on the C stack
rather than the heap (it's only 4 words big).
This means that these methods should not retain a pointer to the buffer
object that is passed in, but this was already a restriction because the
original heap-allocated bytearray had its buffer passed by reference.
