Add FuncUtils:
- getCallFunction:
	Get a suitable function symbol for a specific call
		or return an error
	Return value is of type getCallFunctionReturn
- getArgListString:
	Get the string of a function with function name and parameter list

Modify the process of building the symbol table:
- visitMethodDef:
	Modify the function table with this name
		instead of inserting a single function
	Add the signature of this function into the table
- checkOverride:
	Check overrides of overloaded functions
- isMainClass:
	Issue errors if main function is overloaded

Modify type checks:
- call FuncUtils.getCallFunction to get the symbol
	of this function call

Modify symbol Function:
- toSignature:
	Return the signature of this function
- conflictArgList:
	Check whether this function is conflicted with another