Special handling for special symbols generated during rule binarization

[pczarn]

Benefits:

• can save us space in the prediction matrix
• can save us time looking up medial items when completing generated symbols, because generated symbols are always the first symbol in a binary rule
• can save us space in the prediction transition list, because generated symbols are always predicted in only one binary rule

[pczarn]

Successfully prototyped here: https://gist.github.com/pczarn/c51dd53b5e194467a79ff5592856d14c#file-mini_earley_parser-rs-L327-L331