Move generally useful to_vstruct function into Absyn

It was hidden in the TermParse module.

Move generally useful traverse function to Absyn structure

This function recursively transforms Absyn values in a top-down way,
and it was unnecessarily hidden in TermParse.

Extract Absyn datatype decl. into separate file

This removes redundancy and also gets the standard Poly/ML
pretty-printer in version 5.6 to print the values usefully, rather than
as "?".

extra semicolons removed from (some) .sig files

Get TFL to allow <=> in definitions (of relations or predicates).

Alter the representation of Absyn.absyn and Absyn.vstruct to include
locations. Fill these in in Parse.sml and parse_term.sml as
appropriate. Make sure everything else still builds.

Uh.

Changes to bring about Kananaskis parser. Known bug exists in
treatment of qualified identifiers.

Relatively radical changes to get rid of the preterm type that lived
in parse_term and replace it with a cleaner Absyn type that Konrad
wrote for TFL.