Make it impossible to construct irregular ParallelArrays

[lkuper]

Right now, it is possible to do things like:

var pa = new ParallelArray([[0, 1], [2]]);

In the compiler, this will cause createFlatArray to emit a complaint:

"flattenArray:" "shape mismatch: level 1 expected 2 found 1"

but it won't actually fail; it will go ahead and create pa. Such a ParallelArray is pretty useless. We should probably just prevent them from being created in the first place. (The tutorial already claims that they are "not allowed" -- we should make it so.)

Related: if we get rid of the ability to construct irregular ParallelArrays, then there will be no way (that I know of) to construct a ParallelArray that has false as its flat attribute. All regular ParallelArrays are backed by a single TypedArray and therefore flat: true. So if we disallow irregular ParallelArrays then we can also get rid of any code that deals with flat: false arrays (such as in toString, for example).