postfix format: don't split str's; '_' prefix
postfix format: don't split str's; '_' prefix

In infix_to_postfix(), catch the case when the passed-in object turns
 out to be a string, and avoid iterating on it by wrapping it in a list.
String identifiers will now be prefixed with '_', so they can't be
 confused with any operator.* calls (identifiers allow '.' characters).

rlc2/pygame_maker,rlc2/pygame_maker