Fix some formatting issues
Move code from CallbackHelper to Callback
Move singnal handler creation logic into ISignalInfo
Move Builder.itypeinfo_to_ffitype to ITypeInfo
Add some TODOs
Move Builder.iarginfo_to_ffitype to IArgInfo
Move Builder.ffi_function_argument_types to ICallableInfo and IFunctionInfo
Move Builder.ffi_function_return_type to ICallableInfo
Move Builder.ffi_function_argument_types to ISignalInfo