Attempt runaway stack checker again

[thatch]

(Context: I started writing this, and had trouble with base cases in self-recursive graphs. In the end I just ensured that next-states other than #pop consume a character, which prevents infinite loops but does not help with having a stack whose length is number of characters.)

Basically, the following lexer has a problem:

tokens = {
  'root': [
    ('.', Text, 'string'),
  ],
  'string': [
    ('b', String, 'root'),
  ],
}

for root->string->root loop (there are no pops to ever reduce the depth)