Returns non-zero iff the memory referenced by da represents a valid darray.
Determines whether left and right are equal. They are deemed to be equal iff the following conditions hold:
- They can store objects of the same size.
- They hold the same number of objects.
- If cmp is non-null, calling cmp on each element of left and the corresponding element of right (using cmp_args as the last argument to cmp) returns zero.
(See Darray searching for details of the ‘Da_compare_f’ type.)