make is_rotation more readable by changing x^2 to  and making the function one line.
changed string_is_perm to use sort and isequal rather than character counting
change string_is_perm to is_perm because we have type annotations so we don't need to put them in the function names.
fixed subsets test to be an assert rather than a println.
Merge pull request #1 from astrieanna/master

Requested Code Review