Update Grains exercise

[BNAndras]

https://forum.exercism.org/t/odd-arithmetic-in-grains/8662/23

The example for Grains needs to be updated so the Square and Total functions return a string.

The exercise difficulty needs to be updated to medium.

The "grains on square 64" and "returns the total number of grains on the board" tests need to be updated to use the correct expected values as strings.

[kotp]

Apparently I closed it as a result of misinterpretation involving 269 being merged.

[kotp]

Addressed via #326

I believe this is now "medium", but might be upgraded to "hard" as it uses strings to get beyond the integer limitations, and could theoretically be solved using 8 bit vim, if there were such a thing. Time will tell where the difficulty level should fall.

[BNAndras]

If you’re referring to the difficulty displayed on the site, a difficulty of 5 is a medium exercise. It needs to be 7 or higher to be a hard exercise.