Make experimental kernel's term representation visible to Thm

Thm can then exploit the fact that both the experimental and standard
kernels have a datatype as the term representation. There's not much
that can be exploited there, but it does make it clear that both are
equality types. (To make it clearer, Thm should arguably be a
functor...)

Start removing uses of pointer_eq

It's flakey and seems to cause strange, hard-to-reproduce runtime
failures.

Size functions for terms and types that count the size of the
respective trees. Have written these so many times that it seems
useful to have them in the kernel and available for all. (The
term_size function doesn't include the sizes of the attached types;
haven't ever felt the need for this.)

Rework a bunch of code in the two kernels so that they can share even
more stuff in src/prekernel. In particular, the latter is now home to
a special purpose symbol table that knows all about things being
up-to-date. Also move the final signatures for Terms and Types that
are what is ultimately presented to the user into prekernel so that
both kernels can use them.
In src/0 also move the signatures in Raw-sig into Type.sig and
Term.sig so that {Type,Term}.sml can be compiled in normal mode
against those signatures. I think this makes things a deal cleaner
there.
Finally, witnesses have entirely disappeared from src/0. If we
wanted to reinstate them for whatever reason, they could be stored in
a KernelSig.symbol_table along with arities and base types.

Make mk_thy_type's error message slightly less unhelpful.

Provide a trace variable enabling users to turn off the complaints the kernel is wont to
make about type-variables with unusual names.

More fixity finangling.

Fix for another instance of old constants coming to compare equal with new
constants (caused by del_segment not oldifying constants that are being
cleared out of the theory segment).

Fix for soundness bug; Globals.old changes its behaviour, and is no longer
a reference. It is used in the kernel to do semantically significant things
so can't just be left to a user to change arbitrarily.

Update implementations of terms and types so that they behave like the dB
kernel in their treatment of old, out-of-date constants. Also fix the
Term implementation so that out-of-date constants undergo the name change
to "old->name<-old" that happens in the dB kernel. Previously this
did not happen with Terms.
Now, both implementations store pointers in their internal symbol
tables, rather than records of information that included an out-of-date
pointer. This means that when a pointer is removed from the symbol
table, what it's pointing at can be updated with a changed name.

Update so that this works with the new ML execution code.

Some efficiency improvements in this directory. Code now runs build
about 20% faster. There remains a horrendous time blow-up in
Hol_datatype, which I'm working on. I think it may be subst's
potentially exponential behaviour.

Thanks to an enlightened Internet cafe in Perth running DHCP, I can
finally check this in. It's a name-carrying implementation of the
kernel, and should be a drop-in replacement for the existing code.
To make the replacement change src/0 in tools/build.sml to be
src/experimental-kernel.
Sadly, the code is not as fast as I'd like just yet. In particular,
the benchmark example is 20% slower. I don't know quite why this
should be, but I'm looking into it. The performance of the checked-in
code will be even worse than it should be at the moment because I've
used Profile-ing throughout. (The most called (potentially expensive)
function in the kernel? Term.compare it seems.) This profiling seems
to cause a strange Time exception on big builds, which has stopped me
testing the new code on the netsem examples (which are 90% of the
motivation for the change---argh).
Anyway, if anyone feels the urge to fix obvious bugs, please feel
free. Though access to the 'net for my Linux partition is a tad on
the tedious side, I can read my e-mail at home, and this means that I
can see messages to hol-checkins without any difficulties.