Extend unicode-grep to check for more style violations

Now has four tests:
- TABs
- trailing whitespace
- unicode
- line length > 80

Internal documentation notwithstanding, this doesn't currently scan for
missing newline on last line because TextIO.inputLine masks that.

Will rename to hollint or similar in subsequent commit.

The current regression test sequence has the line length test turned
off, but I am keen to turn that on soon.

whitespace cleanup: consistent use of tabs and spaces

Modify mllex so that it doesn't emit code with trailing white-space.

Make mllex skip carriage returns as whitespace.

Fix missing paren introduced by previous check-in.

Encode illegal characters so they print nicely when mllex reports them.

Corrected an error message emitted by mllex.

Extended the mllex regexps to allow ] inside a character class (escaped with a backslash).

For example [[\]] matches any square bracket, and [^\]] matches anything except
a closing square bracket. Technically this was possible before, but the new
language is easier. The examples above in the old language would be [[\093] (or
\[|\]) and [\000-\092\094-\128], and these are still acceptable.

This change breaks code where [xyz\093 or [xyz\] is intended to mean [xyz], but
I doubt anyone does that.

I think standard regular expressions also allow ^ and - to be escaped inside a
character class. Mllex will still treat [\^x] as [^x], but that can easily be
changed if desired.

Make our copy of mllex use the 2002 Basis Library.

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).

Make the organsiation of the mllex source-code a bit cleaner so that
the polyml version can be built more easily. If you have polyml
installed nicely, it should be possible to get a poly version of mllex
by just going
make -f Holmakefile mllex.exe
in the tools/mllex directory. The tricksiness in Holmakefile is more
cute than useful, as it's not used by Holmake.

Get mllex to cope with actions that include strings with unbalanced
parentheses. (Code taken from SML/NJ's mllex.)

Slight change to allow the product of mllex foo to be compiled by mosml
without having to use the -toplevel option. Also a "fix" for an off-by-one
issue that I think is a bug.

A MoscowML compilable version of the "standard" mllex tool, as used by
both SML/NJ and MLton. The source code is also compilable by mlton,
though this is cute more than useful as mlton comes with a version of mllex
anyway.