This lab will be relatively short, as it is a review of concepts
covered to this point.

All problems should be functions within a single file, with main being
a menu to select between them.

1. Using the method discussed in class, seed the random number
   generator, and output a list of the first 20 integers in random
   order.  You may use any technique you want for this problem.

2. Using iterators, output a string in reverse order.  The string
   should be able to contain spaces, and will be input by the user.

3. Using the STL fill function, fill a vector with true values.

4. Rewrite the sieve of Eratosthenes from the last lab, to include the
   use of fill.  Also, please make sure the sieve works.  This time
   around, no matter how well written the code is, if it doesn't work,
   you will get no credit.