Add stub visit_ methods for function definitions
Add FunctionSymbol class, extract Symbol superclass
Store function symbols in symbol table
Add visit_ methods for simple instructions & function calls
Check symbol type in visit_FunctionCall
Iterate through formal/actual arguments in visit_FunctionCall
Implement type checking at function call
Clean up logs/comments
Capture variable symbols in expressions
Fix error handling for variable/function symbols
Stub upward error propagation in expressions
Cover error handling with tests