A whole mess of small changes intended
at making simpsets prettyprint informatively
in the interactive loop. Very possibly
I haven't updated all the files in the examples
directories.

Changed a couple of misleading/annoying names. The type ssdata is now
called ssfrag (to bring it into line with documentation that talks about
simpset fragments), and the constructor SIMPSET is now called SSFRAG.
It never created a simpset, so the latter was a stupid name.

My last check-in broke the building of the n-bit word theories (should
always try the -selftest option to build to catch this) because the
implementation of relevance analysis didn't believe that SUC a <= 2 ** WL
was relevant to proving 0 < 2 ** WL. The solution is to realise that
even ground terms may end up being treated as variables by decision
procedures. So, there is now an additional parameter to RCACHE : a function
that takes a term and returns a list of terms that should be treated
as free variables in the relevance analysis.

A better cache function, RCACHE that does relevance analysis to speed
up the rejection of formulas that are sure to fail.

A few trivial rewrite rules added to the ssdata for bags.

Changing arithLib to numLib.

Slight re-org.