Terms and types print interactively.

Thms ignore grammars; term grammars print badly

More assorted code tidying.

Splitting Arbnum and Arbint to work around the dependency on HOLPP before
moving into the mosml and poly directories.

Move the pretty-printing facilities around a bit so that both SML
implementations use our own version of the PP structure. This
implementation is called HOLPP in src/portableML. After the kernel's
Overlay.sml is loaded, it is also available under the name PP. This
retains fairly good backwards incompatibility. The one deliberate
incompatibility is to make references to General.ppstream invalid.
This makes us better conform with Basis 2002.

The advantage of this manoeuvre is to allow me to better control what
our pretty-printers do at a low level.

Start to implement the Basis 97 extensions that Moscow ML hasn't got
in order to force our codebase to get up-to-date. It should also
mean less bodging around for the Poly/ML code. I haven't checked that
my changes to tools-poly/poly/poly-init2.ML have done all that is
required yet. Feel free to fix problems arising there (I hope it will
just be a matter of deleting things).

Fixed a PolyML warning about missing op for infix operators.

Many small changes, most associated with boolification or lifting.

Added minimum and maximum functions for arbitrary precision integers.

Extended and revised so that the div/mod functions now correspond to
what is done in ML's standard basis library's Integer structure. In
particular, there is now div/mod and also quot/rem, whose behaviours
differ on negative arguments (div rounds down; quot rounds towards zero).

Fixed a bug spotted by Ade Azurat <ade@cs.uu.nl>; fromString was getting
the sign of integers wrong. Also extended the set of acceptable strings
by treating ~ and - as equivalent at the start of strings representing
numbers.

Division and modulus operations on integers were occasionally returning
-0; something the design was supposed to prevent.

General cleanup in view of the fact that we now have the Standard Basis
library. Hol98 assumes the existence of this!