Minor code cleanup in parser table construction.

Minor code cleanup in parser table construction.
* src/LR0.c (new_itemsets): Use item_number_is_symbol_number.
(new_itemsets, save_reductions): Update for rename to nitemset.
* src/closure.c (nritemset): Rename to...
(nitemset): ... this since the "r" appears to meaningless and isn't
used in the comments.
(closure): Update for rename.
* src/closure.h (nritemset): Update extern to...
(nitemset): ... this.
* src/lalr.c (LA): Fix a typo in comments.
* src/print.c (print_core): Update for rename to nitemset.
* src/print_graph.c (print_graph): Likewise.
* src/state.h: Fix some typos in header comments.