malloc: allow removing memory from named heaps

malloc: allow removing memory from named heaps

Add an API to remove memory from specified heaps. This will first
check if all elements within the region are free, and that the
region is the original region that was added to the heap (by
comparing its length to length of memory addressed by the
underlying memseg list).

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>