implement zeros() by calling calloc

There's a clever trick that we could use to create large zero matrices really fast: mmap the file /dev/zero. This is, in fact, exactly what this "file" exists for. The benefit of doing this are:

1. It's nearly instantaneous since no memory actually needs to be allocated or filled with zeros until it's accessed.
2. You can read and write the memory exactly as you normally would: the kernel only allocates memory pages for you when you do something with them.

Since a fair amount of the time, no one actually touches most of the memory in a zeros array, this might be a big win. On the other hand, the drawbacks are:

1. Trade obvious immediate allocation cost for unobvious delayed allocation cost.
2. Can run out of memory on read/write instead of only on allocate.
3. Doesn't work for anything but zeros(), e.g. for ones().