[plt-scheme] Change to setup-plt

There were several problems in the way setup-plt was crawling the
collections tree.  I fixed that and I made some changes to the set of
"info.ss" fields.  The important changes are:

* `compile-subcollections' is no longer used.  (If you happen to have
  a leftover value, setup-plt will print a warning.)  Instead, *all*
  subdirectories are now compiled by default.  This can of course lead
  to problems, but it turns out that it fixed more things than it
  broke.  (There were certain directories that never got compiled
  previously.)

* `compile-omit-paths' is a new field that specifies paths to skip
  when compiling -- it specifies both paths of files and
  subdirectories.  (But immediate names only, no "foo/bar".)  Also, it
  can be set to 'all which will effectively make setup-plt ignore that
  collection or sub directory.

* `compile-omit-files' is subsumed by the above, so it should no
  longer be used.  It is still read in for compatibility though.  (But
  I removed all uses of it in the PLT tree.)

I fixed everything that needs to be fixed -- added the necessary
`compile-omit-paths' where needed to avoid compiling bogus files, and
dealt with places that needed fixing for a while but didn't cause a
problem since they were not compiled so far.  Some planet packages
might need a similar cleanup.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!