CS 345 Assignment #7

Extend the result of Project #6 to implement a "return" statement.
Do this by extending the Checked type to include a Return case, which
the CheckedStateful monad treats very much like an Error. Then define
a function "handleReturn" that is called in the Call evaluation of a
call statment, with this type:
	handleReturn :: CheckedStateful Value -> CheckedStateful Value
It should be called around the result of evaluating the body of the 
function being called. "handleReturn" will convert Return values into
Good values, and Good values into Good Undefined values. You should add
another case of value to be Undefined, which is the return value of
functions that don't call "return".

Also implement {...} braces for if statments in the grammar, and add
"return Exp" and "undefined" to the grammar.