Fixed output of students->users.rktd

users.rktd is expects to only contain string, but students->users.rktd
could output arbitrary data if the wrong thing was added to a student
struct. Now it coerces things to strings.
Added exam-script, update-student script

The exam script will take an exam grades.rkt file and output a bunch of
stats.

The update-student script will take a list of partners/grader assignment
files and output a new users.rktd file with the students' grader
updated.