initial TypeChecker files
symbol_table
actually commit the symbol_table
change TypeChecker to Resolver
Resolver#initialize
basic #resolve ... not tested very well
make FunCallExp not a VarExp
SymbolTable raises an error instead of returning nil when trying to resolve an undeclared symbol
change error message for SymbolTable's undeclared variable error
initial functionality of resolver
refactor resolver
SymbolTable#add_fsymbol and #get_fsymbol
FunCallExp < VarExp
resolve function calls
minor refactors of Resolver