Provide a way of generating non-pub parsers

[osa1]

Currently we generate structs for pub non-terminals, with a parse method. These structs are currently always pub. So if I define a grammar in an internal module and define a parser in the same module, I get a compile error because the parser is pub but its interface uses a non-pub type (the parse tree).

We should mark entry points of grammars with another symbol and use pub to make the generated parsers structs pub.