The pushback string iterator used to pre-consume
the string and fill in a list, one character per
entry in the list.  Pushing back a character just
meant appending that character to the list.

Now it's a stack of iterators.  Pushing back a
character means pushing an iterator that yields
that character.  This better optimizes for the
general case, where we yield long strings of
characters.

Minor changes:
* The previous pushback string iterator had a
  lurking bug: if you pushed a string of
  len(s) > 1, it would yield the characters of
  that string in reverse order.  Perky never
  hit that bug, because it only ever pushes
  back individual characters.  But still!
* If a line is a triple-quoted string, don't
  ever yield a WHITESPACE token for trailing
  whitespace (if present).  Perky's tokenizer
  suppresses *trailing* whitespace on the line
  in every other context, it needs to be
  consistent here.
* Some doc improvements, mainly in the
  introductory section.