Automatic detection of main rule name assumming it is the first one

[KOLANICH]

Arpeggio requires explicit first rule name. It can be assummed it is the first one - it is the usual convention. But that the user has to provide it is hardcoded in ParserPEG architecture (PEGVisitor is hardcoded), it cannot be changed nicely (without monkey-patching) without changes in arpeggio itself.