ParseItem no longer has a lookahead_sym field; it now represents
the 'core' of a parse item. The lookahead context is stored separately,
as a set per core item. This makes iterating, copying and merging item
sets more efficient, because before, the core items were repeated for each
different lookahead symbol.
Also, the memoization in sym_transitions(ParseItemSet) has been removed.
Maybe I'll add it back later.
Now, instead of adding states to the lex table as they are needed
by the parse states, we iterate over the parse states after the fact
and set up their corresponding lex states. This has the nice side
effect that the lex states are in a more readable order.
