changed how we store values in symbol tables

made a wrapper class to hold symbol table values and changed code
accordingly
NOTE: check out AssignExpr in type checker for all the annoying things
we have to test for (variable not declared, types dont match) - need to
avoid null pointer errors
Also changed all the error reporting to go through our new method, wrote
some of the errors that were just commented.
I think the duplicate visitor is done, check out the declStmt's use of
isField()
not sure what github is doing
wtf
so hopefully this works
maybe this will fix?
Merge branch 'master' of https://github.com/imtibbet/Project3Source