substring function for UTF8 strings

Provide a UTF8-aware padRight function

This can then be used in the testing routines that want pretty output
that might include non-ASCII characters.

UTF8.getChar was too liberal.

In particular, it allowed UTF8 encodings that used more bytes than they
should. Fixed this and added a bunch of test-cases. Also, stopped
getChar from decoding past the maximum allowed code-point.

Thanks to Rob Arthan for the bug report.

Closes #237

Get UTF8 conversion to handle codes above the 16 bit 'BMP'.

Add translate function (analogous to String.translate) to UTF8 module.

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).

Add some simplistic code for decoding and encoding UTF8 strings. Also
add even more naive "character" categorisation functions to
UnicodeChars.