Fix what I think is an off by one error in column positions for
location reporting.

Provide a short-printing form for locations.

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.)