(Try to) Fix left recursion

I've made some headway but there is more.

I fixed the initialization of `involved_set` and `eval_set`, and the infinite loop in `setupLR`. The first test case, parsing only one character, now passes. But the second still fails with this assertion failure:

    backends/packrat.c:85: setupLR: Assertion `state->lr_stack->head != ((void *)0)' failed.