Added toString methods to all of the Expressions except for ForEachStatement, IfElseStatement, and WhileStatement.

The gramar was able to compile whileloops.tscalr, however there are two problems:
1) The main function is executed out of order. (when there are more than two lines).
2) The grammar does not seem to be performing math correctly.
For 2), my productions for artihmetic may be conflicting with the productions for expr, which is to say, I'm not sure which one is actually being used by the grammar and when.