You are given a set<string> containing all of the words in the English
language, all in lowercase.  Write a function that takes in a string prefix,
then prints out all of the words in the set that have that string as
a prefix.  However, if you iterate over an element of the set, you must
print it out (that is, you can't just iterate over the set and print out
words with the right prefixes.  You need to find a way to iterate over
just those words with the right prefix)