trivial output tweaks for vl model command
Merge interface work with Sol
some basic interface tests
Allow documentation fragments in defsection/define

This is a little hack that may give a more convenient way to add a bit of
commentary to a long sections.  For instance, it lets you write things like
this:

   (define frags (x)
     :short "Example of documentation fragments"
     (if (atom x)
	 1
       (+ 1 (frags (cdr x))))
     ///
     "<p>This would be better written as a type-prescription rule:</p>"
     (defthm posp-of-frags
       (posp (frags x)))
     "<p>But this would not make a good type prescription rule:</p>"
     (defthm frags-of-cons
       (equal (frags (cons a x)) (+ 1 (frags x)))))

In the resulting documentation, you'll get these little paragraphs interspersed
with the events in the "Definitions and Theorems" section.

This hasn't been well tested but it seems to work on small examples.
Add new immediate dependency stuff
Merge remote-tracking branch 'remotes/origin/master'
Merge remote-tracking branch 'remotes/jared/doc'
Slight reorganization

 - Move toposort out of VL and into a new centaur/depgraph library
 - Move cons-listp and cons-list-listp out of VL and into std/typed-lists
 - Move some lemmas about intersectp-equal, intersection$, and set-difference$
   out of VL and into std/lists
 - Fix various of hints to deal with the above
 - Rework drop-missing-submodules to use scopestacks
implement transdeps function
merge