Combine LR1 items with same productions

[osa1]

LR1 items:

parsegen/src/lr1.rs

Lines 10 to 17 in f368fac

	#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone)]
	struct LR1Item {
	non_terminal_idx: NonTerminalIdx,
	production_idx: ProductionIdx,
	cursor: usize,
	// None => EOF
	lookahead: Option<TerminalIdx>,
	}

LR1 sets:

parsegen/src/lr1.rs

Lines 80 to 84 in f368fac

	fn compute_lr1_closure<A>(
	grammar: &Grammar<A>,
	first_table: &FirstTable,
	items: FxHashSet<LR1Item>,
	) -> BTreeSet<LR1Item> {

This representation generates a lot of redundant (duplicate) info in states:

1571: {
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "("]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "bool"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "-"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "-."]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "+"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "+."]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "*."]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "/."]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "="]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "<>"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "<="]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "<"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, ">="]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, ">"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, ","]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, ";"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "id"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "int"]
  [LetExpr -> "let" "(" Binder CommaBinder1_Rev ")" "=" SeqExpr "in" SeqExpr|, "float"]
}

All of the items in this state have the same production. We can combine all of these items into a single one with a lookahead set:

struct LR1Item {
    non_terminal_idx: NonTerminalIdx,
    production_idx: ProductionIdx,
    cursor: usize,
    lookahead: FxHashSet<TerminalIdx>,
}

We can either add one more field for whether EOF is a valid lookahead, or we can assign EOF a TerminalIdx.

See #1 for optimizing terminal sets. (FxHashSet<TerminalIdx> in the code above)