Add const vs. dynamic ndim to array Debug format

[jturner314]

The difference between const-dimensional and dynamic-dimensional arrays is fairly significant, so it would be useful for the debug format to include this information. See also #489 for more motivation.

This implementation produces a debug format like this for a const-dimensional array:

[[0, 0, 0, 0],
 [0, 0, 0, 0],
 [0, 0, 0, 0]] shape=[3, 4], strides=[4, 1], layout=C (0x1), const ndim=2

and like this for a dynamic-dimensional array:

[[0, 0, 0, 0],
 [0, 0, 0, 0],
 [0, 0, 0, 0]] shape=[3, 4], strides=[4, 1], layout=C (0x1), dynamic ndim=2

(Note the addition of const ndim=2 or dynamic ndim=2.)

Is the term "fixed" better than "const"? I chose "const" because the number of dimensions is a compile-time constant, and Rust often uses the name "const" for this.