This exercise asks you to add limitted input/output to FUN.
Specifically, to extend the environment-based definition of FUN to
include two more "special" names, "read" and "print", whose semantics
is as follows:

- "read" evaluates to the first item in the stdin, which is expected to
  be an integer.  For simplicity, we assume that "read" only reads
  integers and only from stdin.

- "print" behaves like a function which takes one argument, evaluates
  it, and then prints the result to stdout; the print statement itself
  evaluates to the printed result.  For simplicity, we assume
  that "print" can only print integer or string results.
