array(length0, length1, length2) -- three-dimensional immutable array

array provides three-dimensional immutable arrays. These are actually
one-dimensional immutable arrays with an additional access function with
three index parameters.

pre
  safety: length0 ≥ 0
  safety: length1 ≥ 0
  safety: length2 ≥ 0
  safety: length0.as_i64 *? length1.as_i64 *? length2.as_i64 >=? 0
  debug : length0.fits_in_i64
  debug : length1.fits_in_i64
  debug : length2.fits_in_i64