Variadic version of cartesianProduct.
Fix compile error.

Add unittest.
Disable unittest that eats too much memory.
Expand docs to showcase 3-argument cartesianProduct.

Use same code in unittest (it's good to make sure examples actually
work! :-P)
Showcase precise ordering of output.
Merge pull request #1120 from quickfur/multi_cartprod

Implement variadic cartesianProduct