Rename ATVAL to F_ATVAL for consistency
Format string sub-expressions are enclosed by %( %), not %{ %}
Support +"" format string
Change meaning of SEL_UNIVERSE

- since universe doesn't require TOS to be valid, make SEL_UNIVERSE a
  pure popper.  "universe" is then translated as SHF_DROP
  SEL_UNIVERSE, "+universe" as SEL_UNIVERSE.  This way we can actually
  pretend there's a +universe at the start of each query without
  having to handle the stack effects specially.
Shuffle tree type constants around so that they make better sense
Add a stack effect checker

- This walks through the program, looking for stack underruns or other
  stack effect violations.