Implement Default for owned arrays

[bluss]

zero-dimensional arrays (using () or Vec::new() as the dimension/D type
parameter) will have 1 element in the default state.

nonzero-dimensional arrays (using Ix, (Ix, Ix) and so on) will have
0 elements in their default state.

Fixes #204

[bluss]

ok @SuperFluffy, the reality of zero-dimensional arrays say that these semantics are needed for at least some arrays.

Is it too insane to start with either 1 or 0 elements depending on the D parameter? We could make all arrays have exactly one element too, but then we need to allocate a vector in the common case.

[SuperFluffy]

I think your solution is a good one. Even though I'd prefer to have the 0-dimensional case to have 0 elements as well, though that's obviously not going to work. I am strongly opposed to the other arrays having a default element... I can't articulate why that is, but I'd rather have the 0-d case be a special one.