Move pretty-printer's notion of unsafe variable name to Lexis

Also now call it a "clean" name.

Remove Globals.priming.

Also remove references to it in code that isn't being tested in the
regression checking build-sequence, and so is probably bit-rotted.

Provide numvariant version of variant, which behaves like

with_flag(Globals.priming,SOME "") (variant avoids)

Document change/incompatibility in release notes.

Fix subscript nameStreams to start at 1 rather than 0

This was broken by f42dfacb21e and proofs do depend on
this (examples/ARM/arm6-verification/correctness/correctScript.sml in
particular).

Reimplement variable name-gen operations in kernel with sync-vars

Make Lexis.ok_sml_identifier reject SML keywords.

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).

Implement a new pretty-printing/parsing notation for character literals.
Use the SML notation for this. For example, #"f", and #"\"". This notation
was easy to implement because lexing could piggy-back on the handling
of strings. I also think that compatibility with SML is a good thing.
Changing the notation should be easy enough if we do decide to do this.
Pulled the character pretty-printer out of stringSyntax as well.

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.)