Removed trailing whitespace from all .sml and .sig files.

This affects over 900 files and was done using emacs's delete-trailing-whitespace
function in batch mode. Building the system with Poly/ML and Moscow ML seems to
work, so I'm hoping these changes don't break anything. Please complain if
they do!

Additional functionality for abbreviations.

1. The labels library has been merged into
markerLib and the src/labels directory eliminated.
2. Have added markerSyntax structure.
3. Operations on abbrevs used to all be in
structure Q, even those that didn't deal
with quotations (like UNABBREV_ALL_TAC). Now
markerLib provides all abbrev. operations and
these take terms. Operations that benefit from
quotes, e.g., ABBREV_TAC, MATCH_ABBREV_TAC, etc.
are still to be found in Q.
4. Abbrevs are kept reduced by the simplifiers and
there is a REABBREV_TAC which can be separately called.
This sorts abbrevs topologically, so that they get
restored in a sensible order.
5. For situations where one wants to get rid of all
abbrevs, apply a tactic, and then restore the abbrevs,
there is WITHOUT_ABBREVS: tactic->tactic.
6. Have added a simple topological sort to Lib.
7. Updated proofs in the src directories, and also
in examples/lambda.