lots of refactoring
Merge branch 'master' of https://github.com/person594/cs153

Conflicts:
	scc1401/src/retree/RetreeUtils.java
	scc1401/src/retree/expression/AssignmentExpression.java
	scc1401/src/retree/expression/ConstantExpression.java
	scc1401/src/retree/expression/DereferenceExpression.java
	scc1401/src/retree/expression/Expression.java