[yxnisako] Base 26 checksum

Create an error correcting code or check digit for base 26, detecting mistakes that humans tend to do, like swapping adjacent letters, like the Verhoeff check sum for numbers.  Maybe adapt Verhoeff's idea for the dihedral group of order 5 to order 13.

Better might be taking input in base 27, allowing one escape character, and emitting a base 26 check letter or letters.