Fix left-recursion with packrat

[pesco]

This makes the following test case (leftrec-ne) work (parse "aaa"):

S -> S 'a' | 'a'

It does not work with (leftrec):

S -> S 'a' | epsilon

To be specific, it always succeeds with an empty parse (the second choice).
I currently think this is actually a case that Warth doesn't work for. It might be a degenerate form of the ones discussed in

http://tratt.net/laurie/research/pubs/html/tratt__direct_left_recursive_parsing_expression_grammars/ .

In fact, I cross-checked against the Scala implementation and it appears to exhibit the same behaviour with the following test program (compare the results or lr vs. lr1):

import scala.util.parsing.combinator._
import scala.util.parsing.input._

object P extends RegexParsers with PackratParsers {
  def a : PackratParser[Any] = elem('a')
  def eps : PackratParser[Any] = success("eps")
  lazy val lr : PackratParser[Any] = (lr~a) | eps
  lazy val lr1 : PackratParser[Any] = (lr1~a) | a

  def input = "aaa"

  def test() = parse(lr, new PackratReader(new CharArrayReader(input.toArray)))
}

P.test()

[pesco]

Link dump:

• Failing use case with the go bindings: https://gist.github.com/jrozner/8033233
• Alternate variant of Warth's method, using exceptions: http://walpurgisriot.github.io/blog/2013/12/13/indirect-left-recursion-in-packrat-parsers.html
• Another alternate implementation: https://github.com/axilmar/parserlib/blob/master/LEFT_RECURSION.txt
• Warth et al.'s paper: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.81.293
• Scala's implementation: https://github.com/scala/scala/blob/v2.10.3/src/library/scala/util/parsing/combinator/PackratParsers.scala

[fbz]

The docs should be updated to clarify the cases that left recursion can and can't handle, but yeah this is ready to merge.