Work towards use of PolyML.pretty type for most pretty-printing

Compiles up to src/parse under Poly/ML

More assorted code tidying.

Set up Poly pretty-printer for HOLset type.

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!

Use Red Black sets for everyday usage

An alternative implementation of sets: randomly balanced trees

Pretty significant "refactoring" of the early source code. All the stuff
that was in common between 0 and experimental-kernel moves into a new
prekernel directory. This can be thought of as the kernel utilities
directory, full of code that is not quite portableML because it's fairly
HOL specific, but that isn't really part of the core implementation of
types, terms and theorems.
(Also add a new pair_compare function to Lib.)