Rename the Array module to 'CArray' and the array type to 'carray'.

This is not backwards incompatible but it's probably a better default, since
it doesn't hide the standard Array module or array type.  It's still possible
to rebind CArray to Array to enable the index syntax:

```
   let module Array = CArray in begin
      a.(i) <- a.(j) + y
   end
```

Fixes #115.