Fix HOLsexp selftest given change to API (fromList -> List)

Implement s-expression type for use in serialisation applications

Fix UTF8.substring to work on start indexes <> 0

Implement Portable.mapfilter to evaluate from left to right

Thanks to Konrad for noticing the misfeature of the previous
behaviour.

Fix blemish in output from portableML/selftest.sml

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 new selftest (from db32edaf6) to actually compile. (Sheesh)

Self-test for important "non-idempotence" property of HOLsets.

In particular, if the set's comparison function is an equivalence
relation, then check that adding x0 and then x1 (where x0 \equiv x1),
results in a set containing x1, not x0.

Fix bug in Redblackset.isSubset (empty sets were only subsets of empty sets)

This came up while investigating github issue #113