avl: Use definitions from order module

The AvlCompare type defined in the avl module is identical in signature to
the compare function used by qsort() and bsearch().  That has a common
definition in the new order module, so use that rather than defining its
own.

In addition use the standard comparison functions from order where possible
for the avl test code.

Cc: Joey Adams <joeyadams3.14159@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>