Instead of taking an &str take into<GString>

[JAicewizard]

This should be completely compatible with the old generated code, however I have not looked at all cases. Cases I did explicitly look at are:

• &str parameters
• Option<&str> parameters
• function parameters that take &str still take &str

It would be ideal if there was some kind of &GStr, to avoid hacking around ref_mode like in this patch.