Use decorator design pattern

[yossigil]

on fanyEBNF.
nullable, get first, follow, etc. should be in decorator(s)

[OriRoth]

@yossigil Done in 2938d28. See new hierarchy in classes EBNF and FancyEBNF. Language test cases now use EBNF instead of FancyEBNF. If it is to your liking, close this issue, comment otherwise.