Solve Problem 11.

getLargestProductIn(grid) isn't that bad for a first pass at the solution.
But I'm not too keen on how I had to manually project the 4x4 sub-grids.

Since I'm taking subgrids starting from the top-left corner, I can count
from 0 to 16, where 16 is 4 less than the length of sides of the grid.

I'm really impressed with the extension methods that LINQ provides,
even if some of them are named funny:

* Select() instead of map,
* Aggregate() instead of reduce,
* SelectMany() instead of flatten?

But hey, a rose by any other name... still smells pretty sweet!

Step 3 of Problem 11: Inline the helper method.

Also remove the test that covered it, since it's going away.