Get number of allocated members in array
#include <array.h>
int64 array_length(array* x,uint64 membersize);
array x; int64 members = array_length(&x,sizeof(t));
array_length returns the number of initialized bytes in x, divided by the size of t. In other words, array_get will succeed for positions 0 through array_length-1; it will fail for position array_length.
If x is unallocated, array_length and array_bytes return 0.