isNullable is exponential in certain degenerate cases

[djspiewak]

lazy val a = b | b
lazy val b = c | c
lazy val c = d | d
// ...
lazy val z = a | a

Example courtesy of Michael Adams. I'm pretty sure this requires 2^26 operations to compute, since tracked is unshared between branches. Unfortunately, the fix is not as simple as extracting tracked to be shared by both branches, since doing so will cause nodes to not be revisited when their values have been subsequently determined. An example of where naive extraction will fail:

lazy val a: Parser[Any] = b ~ c
lazy val b: Parser[Any] = c | () ^^^ ""
lazy val c: Parser[Any] = b | "x"

a.isNullable must beTrue